This was the first module I got - from Lars, a friend of mine. Actually, I didn't even own a keyboard to control it at the time - I borrowed one. I used it when recording with one of my bands in a studio, and it did pretty well.
So, what can be said about the Proteus? Well, it's a very all-round rock/pop machine. It has 32 voices (but really, most sounds use at least two voices), and it can listen to 16 MIDI channels. Other than that, it has a quite nice feature in that it has two extra stereo outs, and two stereo returns (the stereo returns get mixed with the main stereo output), which is quite useful if you want to have some effects on the sounds (which you do, since it has none built-in except for a chorus which is implemented as doubling with detune, and thus uses additional voices).
Since I had the luck of getting a module with the +2 expansion, I also have a bunch of really nice strings and woodwinds, and I must say that the quality is far better than most modern GM, GS or XG modules. Since my box also has the XR expansion (for eXtra Ram), it has 512 preset positions, of which 384 are user-alterable. That's pretty nice, don't you think?
Anyway, I don't think I will sell this one very soon, because it really does a great
job at the sounds that none of my other modules do can do.