It's as perfect as the real thing. You can use the transfer pack, rumble packs, memcards, and so on. Plus it's easier to defeat the end boss of CBFD, and Bowser in mario64. Possibly others that require you to turn the stick too.
It depends, do you want to only emulate cause then get an Adaptoid. Or do you intend to play PC games too. If this is the case, Adaptoids are not always quite as compatible so you should get a PC Pad, I reccomend the Thrustmaster Dual Power 2.
I own two, the only problem I have had is I havn't been able to configure the VRU for Hey You Pikachu! That's the only problem, otherwise it works perfect, perhaps I am doing it incorrectly, I will try it with the newest verson of N-Rage...
I have one and have never had any problems in it. It is just as compatible as any other pad, even for pc games, since it is a standard HID input device.