I like that these things keep the price of the legit hardware from spiraling out-of-control (I hate competing with people going through a temporary 'nostagia kick'), but yeah, as Howard said, they're usually a little spotty on compatibility with any cartridges utilizing special chips (Castlevania III, Yoshi's Island, Starfox, Virtua Racer, etc.)
These machines are decent if you have a box full of carts in your attic and you want to give them a play without buying a bunch of dedicated consoles but, otherwise, have no serious desire to collect.