Which one did you download? There isn't an official Xbox Live app from Microsoft on the Android Marketplace, so any of them that you see are from third-party developers. Just be sure when downloading an app (this goes for all android or iOS apps) that it
has a good rating (3 stars or above) and read some of the reviews from other users if you are unsure about it.
I've been using the 1337pwn.com on my Android for about 2 years now and have never had any problems. It will alert you when your friends sign on and you can see your friends achievements and gamerscore.