Nestopia for the Mac is a watered down version of the PC emulator. Nestopia on the Mac can emulate most NES games just fine, sure, but you only have two window size options: thumbnail or full screen. I like full screen but it's a bit fuzzy and out of focus. It also makes NES games look super dated.
Nestopia only supports freeze states. You can save an unlimited number of freeze states, but there is no in game save option. You can play around with the interface and the video in Preferences, but there aren't as many options as other emulators.
If you have Emulator Enhancer, which you can use for free but will eventually need to buy for $30 if you want all the features, you can connect USB controllers just fine. However, I'm a bit peeved that the original Nestopia offered controller support free of charge, but that option was taken away in the Mac version of Nestopia.