Your question is worded in a way that might include any kind of collection, Roger, but I'll address the film score issue first and foremost.
The issues I see are:
- why are you interested in film music?
- why do you collect film music?
- what sort of collecting is it? compulsive? discriminate? for aesthetic reasons only? partly for speculation?
- what are your criteria for buying? enjoying? keeping? discarding?
The prime motive is the enjoyment of music, mostly film music.
The prime criterion is-- the same.
I don't buy something because it's limited & rare, nor to make money-- I can understand some people doing this in a reasonable way to finance part of their film music collection, mind you; it's "blind" speculators I don't like (people who don't even know a thing about film music, except that some releases can make them a fortune, and who buy limited releases by the dozen).
My first purchases were "the big ones" (Williams' blockbusters) and action scores (Goldsmith, Zimmer) (like Thekongologist-- couldn't think of something simpler to type, eh? :p ), ).
It turned out that Goldsmith was the one I liked the most; whether it's a good movie or not, the score is always (well, 90%) (at least) good; I don't like Johnny's early scores, on the other hand-- I am not interested in Johnny Goldfarb, for instance.
I am thus a Goldsmith semi-completist; I didn't jump on every release, and only got Along Came a Spider a few years later, for example. I can wait for a bargain on, or even do without, such a thing as-- is it City Hall? That movie with Kevin Bacon and Gary Oldman; I didn't like the score too much watching the movie; maybe it would fare better on CD, though I don't rmember reading anythng good on it; but then, the much-reviled Mr. Baseball is definitely not a bad score; and, like BigMcGyver, there might be something to my liking; but it's not a priority, anyway.
Thanks to Goldsmith and other composers, exploring their softer sides, my tastes have expanded; I still won't play Planet of the Apes daily, but I can appreciate it more, and I absolutely love Star Trek: The Motion Picture. I also started exploring the Golden Age.
My collection obviously started expanding with my becoming a teacher, thus earning regular revenues. The pace picked up a few years ago, somehow, with tastes expanding. I bought my first Schifrin and Scott scores in the past couple of years (1-2, possibly 3 fro Schifrin).
Beforehand knowledge of the music, mostly thanks to the movie, is of course the best and surest way to plan purchases, but I have not seen a majority of the movies whose score I have: either because I missed it, because I'm not interested, because I still haven't gotten a chance, or often probably won't ever have a chance of seing it.
Samples help a lot. Reviews, descriptions and recommendations can be quite helpful, too; there may be disagreements, but overall I tend to agree with James Southall, for instance, and Doug's short descriptions give a good idea what to expect.
A lot of it, however, is intuition-- "sixth sense", like Thekongologist said. Fortunately, I'm usually right.
Now to quantify that "usually"...
With 897 CDs (three received this morning), I have had very very very few major disappointments.
Morricone's aforementionned The Island is one; the samples were good and intriguing, but they truly were the best moments; there are a few good tracks, but I could have done without it. The problem was that it was the start of the Varèse Club Frenzy, there were obviously no reviews, and I didn't want to miss something good.
Such frenzies tend to skew things a bit, but overall, I've managed well.
I have nearly all of Varèse's Club releases (the new series, that ie, starting in the 2000's), an my only other disappointment was with The Return of a Man Called Horse, but that's because of my expectations; like other such disappointments, when I return to the score (I haven't yet), I may really like it.
The limited nature has not become a criterion for me; it may spur more pondering than if it were unlimited, but it's not the main factor. I'm sorry to tell you I don't get any of the Small scores, Roger.
I would have to count, but roughly, I believe the number of scores I have regretted buying must be around 1-2%, out of nearly 900-- a dozen CDs? It sounds a bit many to me; I don't think there are that many; it's more likely a maximum.
I still have them, though. Some parts are interesting. Or I may discover later I like it more than the first time. Or maybe someday I'll trade (not sell) it or give it.
I haven't played Bite the Bullet in some time; I don"t remember there being any samples, and anyway I expected something different. I believe I'll be able to appreciate it a lot more now-- I just haven't thought of playing it to check since then; you need time to forget your expectations, your disappointment, the music, in order to really rediscover it on a fair ground, for itself.
Surely, with that many CDs now, a lot of them don't get played for some time: I play other CDs, I play the new ones, I play some several times, for a long time, regularly, ...
Copying several scores and tracks as mp3s onto my laptop helps rediscover some "old" CDs. It also changes the relation to music a whole lot: it's so easy; whereas, without being excessively lazy, the idea of interrupting my work, getting up, browsing through the CDs, searching, deciding, ..., can be deterring, so that the simple solution is to pick one of the oft-played ones lying around at arm's reach.
Another case I haven't seen mentioned yet: re-releases.
Case in point: Papillon, which I received this morning. I already have the first release; that's why I didn't order this new one immediately; four years later, having played Papillon again this year (after a long while), and copied the theme on my computer, I felt it was time to upgrade to the remastered, slightly expanded version (the sound does seem clearer), I ordered it for my birthday (such occasions are always such perfect excuses

) (but amazon.fr sent me my order a good week too late, though, despite my having planned & placed it for a month).
Some re-releases warrant an upgrade, some don't, or at least not imediately.
Oh, another thing, of the psychological kind.
I like to have something left to listen to. Something new to read, to watch, to listen to.
It might be partly because I (my parents) could not afford everything as a kid; also because I tend to keep the best for later.
In any case, I like to have something in store for later; it's-- reassuring.
I won't read the books I received in this amazon box today not next week nor the week after, ..., for instance, and not just for chronological reasons (volume 2, ...). I prefer to wait a bit, and I have a good stock of things to read. Things to listen to, as well. And things to watch.
A few brief points on other sorts of collecting.
Besides literature, I like comics, especially Disney comics-- actually, Thekongologists, I think the parallel would be between film music lovers and Disney comics lovers (rather than comics in general): millions of people like comics (whether American comics, European bandes dessinées, Asian manga, ...); Disney comics are read throughout the world, but we are not that many to know, like, read, collect, the most classic ones; and even then, there are niches.
Since "discovering" Mile High Comics two years ago, I have been ordering regularly from them to complete my collection. The criteria are a bit more complex than for CDs. The issues containing stories I already have, often in superb albums, are low in priority; but then, I do get some of these, because I love the cover, or because I know there are interesting notes and features in the issue.
As for other comics: I rediscovered superhero comics due to the first Spider-Man movie; I discovered Alan Moore through From Hell, and from there, such authors and artists as Neil Gaiman, Rick Veitch, ...; I finally got to read The Spirit.
A good deal of my comics are also horror comics, and adaptations of literary works or movies (which might themselves be adaptations). The reason is twofold: for pleasure; for work, as I'm interested in the problem of the adaptation of literary works (into other media: film, comic, music) and in comics studies. I would also like to do something on film music (so I have to know and thus buy certain scores), but I'll be limited by my lack of (technical) musical knowledge.
I'll end here and leave DVDs for later discussion; I probably could / should have done shorter than this already, sorry.
If you want the gist of it all, here it is:
- I buy what I strongly think I will like
- out of almost 900, my misses have been very few
- with so much to listen to, it's tough giving all a fair amount of playing regularly; also, I really know by heart my first 100 CDs, so I don't feel like playing them that often
- I keep it all; I may find I like something later, because I've changed, because of my expectations, ...