One reason they're so popular with the RF crowd is they give great performance at a modest price. Most of them seem to hold their value pretty well, so you can buy one to try it out and if you don't like it, sell it for most/all of your money back. I have the 21/4, 25/4 and 35/1.4 lenses. Build quality for the most part is very good.
The general consensus is that they have a more "modern" look with a bit more contrast than some of the older classic Leica lenses. They also make some crazy fast glass like the 35/1.2 or the 50/1.1, if you're into low-light shooting. The 35/1.4 is my "go-to" walkabout lens on my Leica M2 - perfect size and good speed.
The Zeiss ZM lenses (also from Cosina) have very good reputations as well. More expensive than the Voigtlanders, but optically better corrected and also a more "modern" look with high-contrast, sharpness and better flare correction than even the Leica lenses. One benefit of the VC lenses is many of them use the same 39mm filter size as Leica, if you have a mix of the two brands. I think the ZM lenses use 43mm filters.