What OneTuz said was perfect, people misuse it way too much.
And it also can be really dependent on the guy you are playing defense next to. Ibanez was creating less outs in left field last year because Victorino kept calling him off. So for a lot of those outs, it looked like Ibanez couldn't make the plays. But in reality, he just had an aggressive left fielder. The same goes for Gardner and Granderson in the Yankees outfield. Granderson doesn't call his corner outfielders off that much and just lets them make the plays. Unfortunately, he looks like a light fielder when that happens, and he is playing next to two very plus defensive ball players.
UZR has it's flaws certainly, and it isn't the only defensive metric to suck. But as long as you know what you are looking at and can use the information correctly, it can be a useful statistic. Usually when paired with other defensive metrics it is a lot better.
Cards Sig Game (ranks right after list)
11-4 (18 starts)1st
3.2 fWAR 1st
5.1 rWAR 1st