I've never bought from Riv, but I've made many carefully researched purchases from both Velo and and RH (formerly Compass). So, what do I think of them? Both have been very fair to deal with and the customer service has been excellent. Not every product in their respective line ups fits my needs or my tastes, but those that do have earned my return business. I read Jan's magazine front to back. It's one of the only publications I can say that about. The RH tires are definitely pricey, but after having tried a set I was sold. I've been through two pair of 35's on my Boulder; there's a pair of 38's on my International, a pair of 35's on my Carre, and 650b x 42 on my L'Avecaise. Yeah, they make that big a difference to me. I get my fenders and other bits and bobs from VO. I especially like their rando bars, which fit me very nicely. I recall building up the L'Avecaise and getting to the final stage, installing the fenders. I'm pretty good at doing a solid install with good fender lines but on this occasion something slipped, my punch drifted, and I scratched the hell out of a black fender. Here I was with a shiny new bike and a bodgered up fender. My bike fund was nearly dry too, and purchasing another pair of fenders just to leave one hanging on the wall insulted my innate sense of cheapness - uhh, thriftiness. I called VO to see if anything could be done; they totally commiserated with me, sold me a single fender, and it was in my hands two days later. It was a small gesture, but very much appreciated... and definitely a great investment in a customer. By contrast, I purchased a pair of trail runners a few years ago. Two days after the purchase, the soles began to separate from the $150+ shoes. I called customer service, only to be told that I'd have to send the shoes in to the manufacturer, pay for shipping to and from in advance, and then they would evaluate the shoes to make sure it was an actual defect. They were "pretty sure" I'd qualify for a discount on a replacement pair of shoes. Needless to say, I never purchased another pair of their shoes again.
Back to bikes though: This thread mentions VO, Riv, and RH - but overlooks Boulder, another good company. Maybe not always as responsive as the others, but they've taken care of me and have a pretty amazing inventory of original parts.