I'm a bleedin' cheapskate... yet I love to hoard video games... I'm one of those 'holes that feels Obsessively Compulsively Compelled By The Power of Dog to get my hands on every cool looking game. As such, my backlog of Cool Games I Own and Still Haven't Played (aka 'CoGIOASHaP's) is at about 50.

So in Feb/Mar of this year I signed up for 4 trading sites:
1. Goozex
2. SaySwap
3. GameTZ
4. Swaptree

Now, 3 months or so and 93+15+5+38---> 151 <--- games moved (sent by or received by me) in that time, here's my synopsis...

First up -- some nomenclature for those that haven't done this:
Points -- For some sites, a game is worth a certain # of points. (e.g. a $50+ game is worth 1000 points, a $5 game is worth 100 points). When you send a game, you GET those points, when you receive a game, you LOSE those points.
Credits -- Also called 'tokens' -- this is how some sites make money. For each game you receive, you use one 'credit'. You must buy credits from the site directly (see below...)
General -- For all these sites, you can list your Haves and your Wants, and you can lock certain titles to indicate that they're in your collection but you don't want to trade them right now. When listing Haves and Wants, you can indicate if you have the box and/or manual, and some sites let you list the overall condition of the game. All the sites use an eBay style feedback/rating system, where you rate people that send to you.


GOOZEX
Summary: Credits are $1 a piece. Trades are auto-matched by their system, so your Wants will just start arriving in the mail, costing you a Credit but nothing else. When the system find matches for your Haves, you mail them out (you pay shipping) ASAP.
Pros: High # of users, so I have been both getting rid of and receiving games FAST. Since joining in Feb, I've moved 93 games through the system. Overall quality of the games I've gotten has been very good. I've gotten some rare titles (ShinMega: Nocturne and DDS, Fatal Frame, FF VII PSOne!) with no work.
Cons: No guarantees of game quality, and some people's idea of what it means to include the 'box' is shaky. The site's attractive, but overall usability needs some work (too many page reloads when clicking around).


SAYSWAP
Summary: Credits are $5 a piece. Each user has a message board that people can post to. You can direct-request titles to receive (or send!) with specific users. Because of the direct-request and messaging systems, people mostly lock all of their titles to keep the system's auto-match from happening, then post to each other's message boards to request direct trades only, making it a more social-style interaction.
Pros: Some of the more active users are friendly and very willing to trade. Site design is pretty good -- most actions on your inventory don't require a page reload. You can print a custom mailer with paid-postage on it which you fold around your games and mail -- very nice hassle-free mailing.
Cons: Lots of young kids (like 12/13 year olds) that have no clue how to responsibly trade. Lots of scammers (offer direct trades, then never send their game). System's auto-queue is basically useless (most games locked by users). Very few good users to direct trade with. Point values of games are WAY off... (e.g. Nocturne is valued at about $10, which is stupidly low.)


GameTZ
Summary: Completely social trading site that is 100% free (if you don't donate...). You keep your lists of wants/haves on the site, then you have to manually interact with the site, which will help you find people that have your wants and want your haves. You then Offer trades which users can accept/decline/counter. Once agreed, the system helps you track the status of the trade, allowing you to enter postal tracking #s, remember who has sent/received, etc. until the trade is complete. Also tracks feedback on Senders, and provides a nice Bad Trader Report system to resolve disputes between traders.
Pros: Free to use...! Site has a lot of nice features like links to Amazon values, metascore ratings, etc. Ability to add any video game accessory, game, console, memorabilia, etc to the site. Some very active users, and very good feedback system.
Cons: Appears to be a little past its prime -- most of the really active, good traders are at the point where they have pretty much everything they want, so they're slow to trade or get a little 'elitist' on newbies. (that never happens on any other sites... nope...)


SwapTree
Summary: Site that features semi-anonymous, auto-queue-based trading of games, DVDs, music CDs, and books.
Pros: Free to use! Really nice way to mix-and-match trades -- trade games for music, movies or books, or any combination. Good number of registered users usually means you can find something you're interested in, even if not a video game. If you have a lot of older games, you're after older games, or you like trading all these media types, it's a great site.
Cons: Not a good site for game trading -- there's virtually no new games there. Though you can decline matches, you'll get a lot of matches for stuff of little value for stuff of high value (say some cheap paperback book you want in exchange for your copy of Marvel vs. Capcom 2).

Myqui's Synopsis
In short, for video game trading that's quick and pretty painless (provided you have a lot of stuff you're looking to get rid of, too), Goozex is my choice for best game trading site. I've swapped 93 games on the site since February, and have only had maybe two duds (games received in truly crap condition, or games that never arrived).

GameTZ can be lucrative for trading rare/OOP games, though, if you have something to offer, that is.

Then that said, I love to use Swaptree to trade everything BUT games.

Myqui