There is a thread here somewhere about poor battery life. My last phone was an OG Droid and my battery life is much better with the S3. No complaints or downsides for me as of yet.
While there have been a fair share of users complaining about poor battery life, there haven't been nearly as many as there normally are when phones come out. For everything the S3 is packing, it has very good battery life in my experience. If you're coming from an iPhone 4S, you're likely to get less battery life on the S3, but that's because it's a more powerful device. You have a screen that's 1.3" bigger, more powerful hardware running everything, plus 4G LTE. When you step back and consider everything that's in the S3, the battery life is solid.
Here's the existing thread on battery life that LifeIsABeach is referring to: Battery life??
Since we already have a good discussion on the topic there, I'm going to close this thread. Keeping the ongoing conversation in one place is more beneficial to the entire community instead of having information spread out among several threads. Please feel free to ask any further questions you might have in that thread.