YES, I did try it. I heard it was easier to use than OpenX but it still sucked. I was expecting something like Gmail, simple and elegant and fast. But it was anything but! It took me HOURS to set up an ad. That's one of the reasons we launched uniQlicks. Interface-wise, we win hands down. Reliability-wise, so far GAM are ahead - but wait till their network crashes. Then everyone who has GAM ads on their site will find that their entire pages won't load. In our case, if we crash, our clients' pages still load (without ads). Oh, and if your AdSense is disabled, you won't be able to login to your GAM either.
No, it's an ad manager, i.e. an application you use to manage the ads on your site. You can use it to do things like swap ads sitewide, or run certain ads only for certain users, and gather data about clickthroughs. It's SUPPOSED to make managing ads easier... But GAM just makes things more complicated. GAM (and it's competitor OpenX) are designed for large and bureaucratic corporations where ads need the approval of multiple departments, and so on. There are two big players, GAM and OpenX. There are a few smaller players, notably uniQlicks, TrafficSpaces, AdManage.net, and the comatose Adregate.... and that's about it.