I have Prey installed on a 2009 Macbook, a 2011 MBP, and my Android phone. Works fine on all of them. You can "test" them by reporting the device as missing on the website, and based on the threshold set for reporting (10, 20, 30 minutes, etc) it will start reporting back. It works great for our family's needs, however it will only do 3 devices per account, and it was start erasing reports after so long.
Yes, you need to be able to trust the Prey people with your data, but realize what they do and do not have access to, and you should be fine.
Also, as far as RAM/battery hit, I haven't noticed any change, and I have been running it for well over a year on the MB.