There is nothing worse than spending your hard earned dollars on a game and finding out that it sucks. Sometimes you can avoid this by borrowing it from a friend or reading a review from your trusted, friendly blogger. But if its a brand new game, it can be really hard to find a synopsis that isn't sponsor driven. If all else fails, try the demo.
A lot of games make demos and trial versions of their games. These demos are super useful for a ton reasons. They most importantly give the player an idea of the overall style of the game. Demos also provide familiarity with controls, ranking systems, graphics, and so much more.
One thing to be careful of is demos that are really short. Some give the player a very small portion of a level or mission to complete. Sometimes the available section is very representative of the game as a whole, other times it is not. The powers-that-be may have granted access, but its a specialized section. For instance, you could spend the duration of the demo driving around a city, only to find out that the game has very little to do with driving but more of a first person shooter. Or you could be running around gunning people down, but when the game comes out there is quite a bit of puzzle solving as well. So make sure that you do your reading if any is available.
Be careful with multiplayer games as well. You will most likely find the demos to multiplayer games are small chunks of the solo campaign. While the solo may be great, the servers and overall functionality of the multiplayer aspect may be crap.
Luckily, multiplayer games have something even better than a demo: the Beta. Betas are trial runs of video games. The publishers and developers let loose the game on the public basically to see what happens. These testing grounds are incredibly important to the final product. They are able to get feedback about flaws in the game. A lot of times when games first come out, the maps have flaws that are conducive to cheating. There may also be faulty weapons, glitches involving perks, and so much more.
When games run these Betas, it gives them a chance to correct this before the game is actually released and those leader boards start to look cracked. When there isn't a Beta, the multiplayer can be disastrous (I'm looking at you Modern Warfare 2). However, the Beta for Halo: Reach is out but it is only accessible if you purchased Halo: ODST. In my opinion, I think they should have made the Reach Beta available for everyone. It would help test the limits of the game better as well bring in more players. But at least they are smart enough to do a dry run first.
I am more likely to buy a game i tried and loved on a demo than I am a game that I just have to play by ear. It can never totally hurt to check out a demo or a Beta. They are full of useful information. Plus they are a great way to play a game for 30 minutes while trying to decide what to play for the night.