It isn't difficult at all to use E-Mu's Vintage Keys synthesizer. To be honest, I've really only used the presets for the most part, as I haven't done too much tweaking of the sounds that it already has built into it. It does have a whole ton of preset sounds in here, ranging from Moog synth sounds to Fender Rhodes sounds. I don't know too much about changing the parameters here, but if you want to get in depth with it, it might be a good thing to have the manual around if you can find it.
The sounds found on the E-Mu Vintage Keys synth are varied and versatile to say the least. You've got pretty much every type of analog and vintage keyboard sound in here, or at least some form of it. In my opinion, the more synth types of sounds are the best sounding ones in here, rather than the Rhodes and other more traditional type of keyboards. Those sounds are very good sounding as well, but the really synthy sounds are definitely the most realistic sounding ones. I definitely haven't tried out every single sound in here, as there are hundreds to choose from...
While there are certainly a lot of modern day rack synthesizer out there that are probably cleaner sounding than E-Mu's Vintage Keys, but this is one of the first collection of vintage keyboard sounds, so there is definitely a lot that should be said for this. Beyond this, it's still a great sounding synth module, with a lot of different sounds and plenty of versatility. I don't really know what type of price these would command these days, but it certainly wouldn't hurt to have one of these in your MIDI rack.