I have bought quite a few adventures, though I have yet to use most of them.
For me, the 'right size' of an adventure depends on how often your group plays. If I DM'ed a group that played frequently (say every week) I would probably enjoy mega adventures, but as our group plays infrequently, mega adventures just last too long, and tends to get stale. The players (and their characters) need a sense of accomplishment every so often, so shorter adventures work best for our group. If a mega adventure consisted of many different missions, it might fit the bill, essentially being a series of intertwined short adventures. If it also gave the pc's freedom of choice on which missions to do when it would be perfect. It would really be an entire campaign. Off course the theme would have to be interesting

Is "Black Sails over Freeport" anything like this?
I like adventures that are open-ended, allowing the players to choose their actions freely, without breaking the adventure (anti-railroading.) Necro's "Vault of Larin Karr" is a good example of what I mean. It is, in a way, more of a mini campaign setting, wherein several subplots take place simultaneously, that the pc's can take part in, or not, as they choose. It has several detailed NPC's that they can interact with in any way they choose, which will then have an effect on how other NPC's react to them. Very cool. Another major factor is that it's a
setting, which makes it much more reusable.
Now, I'm not saying that more standard dungeon adventures can't be fun, but they must have hooks that will interest different sorts of groups, ie. both 'the good guys' and 'the greedy guys.' The adventure also has to be fairly logical and consistent. If there is a powerful wizard doing something really stupid or a cleric-turned-evil still living in the temple without the other clerics/paladins noticing his evil, there has to be a
very good explanation.
Since our group doesn't get to play that much, I often just buy adventures for my own reading pleasure, and with the hope that I might get to run them sometime

That means that I don't buy adventures that will 'fit in my campaign' but the ones I just find interesting. Since I don't have a FLGS nearby, I can't look through anything before buying, so I read a lot of reviews, mostly here on ENWorld, before I decide what to get.
Hmmm...I'm rambling on here I guess...it is getting a bit late (3 am) so I'd better stop here
Edit, added: The extra rules/monsters and so forth in adventures has absolutely no selling power to me, but isn't it a requirement of the d20 license that every book has to include at least 5% new open game content? It often seems to me, that it is only there to fulfill that requirement.
darklight