My take on the games I've got:
Braid: Screw the haters--Yes, the plot is convoluted and pretentious, but at least it makes you think, which is more than I can say for about three-quarters of the games I've bought in the last few years; yes, its gimmick isn't the most original, but it's all about how it's implemented in that plot; and yes, you don't get a whole lot of gameplay bang for your buck, but... well, that's something you'll have to deal with. Great art style, great music, and a "story" that's at turns both heart-wrenching and mind-blowing... if you haven't already played it, just do it and do it quick, before my hype diminishes it any more.
Gish: As someone who is sick and tired of games where you play as a blob, I was prepared to leave this one to gather cobwebs in my Steam menu after I got it as part of one of those insanely discounted indie packs. However, to be fair, I gave it a go for a few levels, and I have to say that my prejudices were not wholly unfounded: The game's pretty weak. The concept doesn't really bring anything new to the table, the jumping and wall-sticking controls are frustrating at best, and the overall presentation is pretty low-tech--and not in an ironic way. I don't know what it costs solo now, but I doubt it's worth it.
Portal: Considering that I played this one for the first time this year (on the PS3 Orange Box--yeah, sue me), it's difficult to say that I thought anything but "overrated" when all was said and done. Still, it's a unique, funny, clever game, and if you can get past the fact that the spoiler is about as ubiquitous as "Snape Kills Dumbledore", I think you'll have a great couple of hours with it.
Psychonauts: After beating the PS2 version, I can safely say that this one shouldn't be missed--wonderfully demented sense of humor, wild gameplay mechanics and twists, and a great aesthetic. However, as a game made in the days back when everyone was trying to copy the Mario formula instead of the Wii Sports and/or Halo formula, it does suffer from a bit of collectathon-itis, and there's a whiplash-inducing spike in difficulty near the end.
Puzzle Agent: I got this game with Gish and etc., and figured it would be an easy way for me segue into the Professor Layton-style games that seem to be some of the rage these days. If you like this genre, which to my knowledge essentially consists of interactive cutscenes interspersed with brainteasers, then go ahead. All you need to know about my skill level is that I'm stuck on the one where you have to calculate a bunch of people's work shifts.
Super Meat Boy: I want to like this game, I really do--unlockable characters, a great soundtrack, a slick art style, retro throwbacks... it's just soul-crushingly, emasculatingly hard, exacerbated only further by keyboard controls (Just a tip, Team Meat: STFU about the 360 controller. If I had one, then I'd buy the game on the 360). I can only passively recommend it as a game, but I highly recommend it as a sort of theraputic tool to either (a) Give you a breath of fresh air--and some perspective--from all the casual-ready titles populating consoles these days (but mostly Nintendo ones, which this game is curiously not on), or (b) As a way to enhance your skill at any other game you're having trouble with, akin to swinging with three bats in the batter's box.
VVVVVV: I got this one after LD incessantly sang its praises, and... well, I was impressed. The game takes a simple (and, let's face it, well-worn) gameplay mechanic, and does all kinds of crazy stuff with it. Sure, it's hair-pullingly difficult, but respawns are so swift that it never becomes more than an annoyance, and the soundtrack and charm are surprisingly great--plus, there's plenty to do after you beat the main game. Faux-retro is getting old, though.
World of Goo: If you can tolerate occasionally profanity-inducing physics-based gameplay, I can safely recommend this one. Yes, it's another blob-themed game, but the building mechanic is pretty innovative, the humor is sometimes unexpectedly lol-worthy, and the art style is somewhere at the intersection of Dr. Seuss and a skateboard company.
Other: Amnesia: The Dark Descent is great if you enjoyed Myst, but wished it made you want to sleep with the lights on and a gun under your pillow. Also, The Ball is okay, but I literally can't get the [darn] thing to run anymore.
I own but have not played Machinarium and Osmos. Additionally, I obtained both (along with Braid, which I'm saying I played because I originally got the PS3 version) through the "Humble Indie Bundle #2", which if you don't really care about Achievements or other Community integration, I'd recommend over a Steam download, as you can pay what you want and it all goes to charity.