Some apps are just excuses to put ads on your mobile device, and user reviews aren't always helpful at sorting the good ones from the crap. And the star ratings are pretty dubious--I've seen four star ratings for apps that no one says anything good about--so you generally have to try a couple and compare.

Also, a number of apps turn themselves on and send info without user prompting, so you have to keep close eye on what is running--even if you don't care about privacy, they can drain the battery pretty fast...