I ordered a V5 and received it yesterday, and after a couple of hours of initial testing my first impressions were largely positive.
I got it mainly to play/record my electric guitar and some vocals at home. I was previously using my TC Electronics' Impact Twin preamps, which as far as interface built-in preamps go are supposed to be better than most, but cannot of course be compared to any dedicated "hi-end" gear.
After a couple of hours of listening I would say that the V5 preamp sound signature is more on the warm and thick sounding side; it also has a not too focused nor too open sound, and is very well balanced, with all frequencies well represented across the whole spectrum. Low-end is solid and controlled; mid-range and high-end are sweet and round, without any traces of harshness, velvety I'd say. It definitely has character, without being too colored. It has an overall very pleasing sound signature that is kind of a compromise between "colored" and "transparent" signatures.
Needless to say that the V5 out shined the Impact Twin preamps completely. While listening to both vocals and electronic guitar on headphones, the Impact Twin preamps were able to reach loud enough levels, but were quite obviously thin and overly bright. Not a fair comparison I know, but with respect to certain statements that "all preamps sound the same", it shouldn't take anyone more than a good pair of headphones and a good listen.
Other than the sound, well... there is the famous Avalon bling-bling factor, and in this department the V5 does not disappoint either. The V5 looks absolutely gorgeous and is built like a tank.
Overall I couldn't really find anything I didn't like about the V5, and despite not being able to directly compare it with the M5 I would have a hard time imagining that the M5 could really sound 500 bucks better than the V5, especially in view of both units' specifications. My guess would be that the law of diminishing returns would be fully applicable here.