RenAxx fooled many people when it suddenly appeared in the early 00's: it looks simply like a hastily-reskinned RenVox, with the exact same controls. I ignored it for a long time, but I have rediscovered it since then, finding it quite useful for drums, of all things.
The GUI couldn't be simpler; if you can get past the retina-blazing "Tolex" background, it's just a simple threshold/makeup compressor with automatic release control, and a limiter at the output to prevent clipping (just like Renaissance Vox!). The key to the Renaissance compressors' success, I think, is the automatic release parameters, or whatever is going on behind the scenes.
Compression is smooth and transparent, and you can see why this is marketed towards beginners-and-up. There seems to be something about the compression that "fattens" the sound a bit—somewhat reminiscent of Vintage Warmer's release character—that of course works well for guitars, but I have found to be really effective on drums (individual and full busses), and it can make a snare sound fuller, even at higher levels. You can push this thing quite heavily and still retain a good attack characteristics. It's very useful for slapping quickly on an acoustic guitar bus too, say, to gel them together and tame unruly plosives.
All in all, I'd say this is a delightful tool, and when one surveys the plug-in horizon, there are few compressors marketed specifically for guitars, so I think it's a great effect to have at hand when you need it—especially if you're more of a songwriter than an engineer. And if you're put off by its simplicity, just remember: some of the world's most coveted compressors are fixed-ratio/auto-release compressors with simple controls (UREI 1176, anyone? LA2A?), and I think this is a plug-in classic that deserves reappraisal and a higher place in peoples' plugin roster—especially if you got it during a Waves sale, or free with your Waves bundle.