If you are looking for one for a specific application, you only need to make sure that the unit can provide the proper CFM at the PSI your application calls for, or better. Also, consider the duty cycle. If you are knocking in roofing or construction nails one after another. all day long, then obviously you would need something up to that task. For home projects, the requirements are much simpler.
Really, you should probably consider the requirements of the tools you plan to use. Once you have a compressor, a whole new world of tools become available, so start with what you know you will need, and then look at the requirements of tools you might like to use with it down the road. Size the unit based on this (or a little bigger, if you don't trust the ratings) and you will be covered.
I have several compressors, from a 6' tall floor standing unit, down to a tiny Wal-mart special, and all of them do what they are supposed to, and have held up well. It's easy to spend more than what you need to, so it really pays to figure out the real requirements before deciding on the compressor.