I wouldn't buy the Nanocomp. Or let me say, I wouldn't buy the nanocomp again. I have one and it's not exactly what you call transparent. The nanoverb in my opinion, is not that bad, but you would be better off getting a second hand unit of better quality. Like a Lexicon, uh, the Behringer units are pretty affordable too. I've never used either , but I would think they are better than the nanoverb. It's my opinion though, that the nanoverb is more usable than the nanocomp.
Getting back to compressors, I use mine when tracking the bass guitar and also the bass drum and snare. I don't use an extreme setting, and as Green Lantern said, you can't really hear much difference, while you're recording but, it has made a difference when trying to mix the stuff. Before using it, I couldn't get any punch out of the bass or drums without clipping. After tracking,with a compressor, even with the nanocomp, The sounds of the aforementioned are much tighter and much easier to mix. It's like they hit harder. It has helped in this regard. I wouldn't put the nanocomp on the mix however. THe sound quality isn't good enough. It colors the sound, and can add a little distortion at extreme settings. I have gotten away with a little compression on the mix when mixing some of my heavier stuff.