This is just an unprompted plug: I've been impressed with Concept2. I bought one of their old model B rowing machines used from the Northeastern crew a few years ago, and Concept2 has always been great about support, shipping parts promptly and sending free rowing log books. They host online competitions, one of which -- the Holiday Challenge -- has kept me from physically falling apart for a couple of winters. They've improved their online rowing logs, which can provide a sense of competitive community. They seem to be continually innovating and just doing good quality things.