Hi Eric -
I've had a Concept2 for about ten years, have been using it intermittently over that period, but more this winter than in any of the past few years.
It's a terrific workout, stresses the whole body, works the legs pretty hard, and creates an aerobic workload pretty similar to cycling. I use it on days that I'm not riding, to give my legs a bit of a break while still getting a good aerobic workout. I don't know how much it's helping my cycling specifically, but it feels good, and there's lots of overlap between the rowing world and the cycling world, so something there must be working.
Just like riding a bike on a trainer, it can be pretty boring by itself, unless you're data-focused and like staring at numbers on a screen (which I do). If that's not your jam, again similar to indoor cycling, there are various apps akin to Zwift and Rouvy that can give you a more immersive experience, and connect you to others online in real-time.
The machine itself is awesome, built like a tank and suitable for commercial use. There are lots of other rowers out there, but Concept2 is the top of the line. They also make a bike version and a ski version which use the same resistance unit, but I don't have firsthand experience with those.
- Jonathan