Some games are good, some bad, most average. If you're sure they're good, you buy them day one (or shortly thereafter). If they're bad, they hit the local gamestop/used-game retailer just as fast. If you're just not sure, then you wait until it drops in price, or a cheap used copy. Regardless of how you play it, you find most are or aren't worth full price.
Things I paid less than full price for, but would have paid full price:
1) Deadspace: Okay, I'm not big on horror media in general, though I've watched enough to know how it all works. Space horror works on me. Of the horror movies that I've seen, Alien and Event Horizon are the only ones that actually scared me beyond the standard-issue "spring-loaded cat" moments, and I like those movies. Most other horror movies, ehhh... not so much. And even though it uses the Doom-style monster closet spawns, it still has the atmosphere those two movies had. Scared the crap out of me first playthrough, while going through with powered up weapons wasn't so bad. Then I played on the hardest difficulty. Yeah... I ended up listening to a podcast most of the way through, the enemies were so much harder that listening to the music and SFX would've freaked me out more. Either way, if I'd been interested in the game when it came out, I definitely would've paid full price for it.
2) Civilization 4: Got it during a sale on Steam, heard about it because the guys on the Rebel FM podcast (www.eat-sleep-game.com) went on about it for several episodes. Great turn-based strategy game, look up some reviews.
3) Assassin's Creed 2: Just came out (at time of writing), so just look it up. Admittedly, it was a used copy for only $5 cheaper, but, you know.
Games I paid full price for and wish I hadn't:
1) Cross Edge: I bitched about this one a couple months ago. Douchebag friend borrowed it, liked it more than I did, but has also failed to finish it. Haven't gotten it back yet, though I would since I plan on trading it in as soon as I do...
2) Prototype: Fun for what it is, but average game.
3) Overlord 2: Would've/should've gotten it used, but didn't. Kinda quirky and different, but still, a meh game (to me anyways).
Games I bought for partial price and glad I didn't:
1) Eat Lead: The Return of Matt Hazard: Okay, this game is a definite rental. Gameplay is kinda average, BUT the plotline is worth playing through once simply because of how sharp the writing is, with its large number of homages to games and nerd culture, all the way to its incredibly genre-savvy hero, Matt Hazard.
2) Star Wars: The Force Unleashed: Just finished this today (of this writing). Not a bad game, graphics are good, so is the story (to my suprise). But there are enough bugs and wonky controls, among other things, that I glad I didn't pay full price.
3) Mirror's Edge: Liked the demo, played the first couple levels. Haven't gotten around to finishing it, and need to.
Full Price, worth every penny:
1) Batman: Arkham Asylum: Well duh!
2) Borderlands: "You know there ain't no rest for the wicked..."
3) Halo ODST: Screw Master Chief, ODST for life!
I'm probably missing some, but I didn't want this to be too long, especially with this long list of tags.