I used one very successfully for deer hunting this fall. I would consider such things as:
1. Rating of the IR detector (they're not rated all the same)
2. Memory card - either get a camera with a viewing screen or one with the memory card compatible with your digital camera.
3. Get one with IR flash (animals won't see the flash although my regular flash did not seem to scare them off)
4. Compare the battery life - they usually use D cells and these are expensive and don't last all that long.
5. Get one with sighting capabilites - you want to make sure you are pointing the camera at the right spot or area.
You can do your research by accessing the Bushnell or Moultrie sites and comparing their different models and don't forget e-bay as a possible supply source.
Good luck!
Duke1