My brother started out as a game tester and worked his way up to an Associate Producer for Vivendi Games, the parent company of Sierra and Blizzard, so I'll give you my second-hand experience of what it really means to be a game tester.
Sounds like a sweet deal - you get paid to play games! You got pretty decent pay: $12/hr for entry level work, which is better than what you'd usually get, even at most office jobs. But there are drawbacks. For one thing, you have to go to them. For him, that meant driving about 30-40 minutes away. Vivendi's LA base is near the airport, Activision and EA are nearby as well, though I believe that EA's base is in Northern California and they do most of their hiring from that office. And that's without traffic because his hours were really funky: 10PM-7AM or 8AM. He'd work the graveyard shift because they have people testing games pretty much 24/7.
Then there's the actual testing. You have to play the same level over and over. You're not playing for fun, and you're not playing the finished product.You have to look for glitches and bugs, which means you have to do the same thing on every level several times to make sure that it does what it's supposed to every time. And then you have to do something else the same time, every time. You're doing Alpha testing on rough cuts, which could be anything from Half-Life to Barbie's latest adventure. One of the lead testers had a list on a whiteboard. If you pissed him off for some reason, you got Barbie.
Then there's the crunch, working overtime and late hours to do Beta testing out a game as much as possible before it's released. It's a little more bearable because they're supposed to have worked out all the glitches and cleaned up the graphics. But every glitch you find has to be documented exactly so that they know where the coding is messed up, and you cringe every time that you find one because you don't want to be the sorry bastard that brings it up since it could be something that takes seconds or months to fix.
Of course, the question of job security always looms overhead because if you mess up bad enough, there are hundreds of eager gamers out there that believe testing would be a dream job. And when they're done with that game, they may not have another one for you to test until the end of the next quarter. You might be just a contractor that comes in for the Beta testing.
Luckily, my brother had a degree in Psychology with a minor in Computer Science. He was able to move up from testing into Q & A, Lead, and now into the production aspect. But he works long hours and often 7 days a week, especially with his latest project
TimeShift. As I wrote about in that thread, the game has been pushed back and completely overhauled several times in the last few years. I believe he's been on it for about 2-3 years straight. After the game's released, the company will give him anywhere from a week to 3 months to recoup, having him support other stuff that's nearly ready to be released, before sticking him on another developing game.
So there's a second-hand account of what life is like as a game tester. I think I'd rather be a reviewer than a tester or designer. I'd wanna work for IGN or GameSpot and play games that are already finished, or nearly so. I'm sure if I could get in good with the editor, then I would only have to review the games that I want to and stick the noobs with the crappy ones.