I like the rower, although I don't know if the kinds I've used are that exact variety. I wouldn't want it to be my only workout, though.

Although cycling is my main cardio the Concept2 is a great piece of kit - closest to a full body cardio workout you can get really. Add some sit ups and push ups and you have just about done your whole body.

You can get a lot of work done in a very short time. It's my go to piece of equipment when I'm short of time. Also find it really good when I feel I need or want a warmup before lifting.

2k flat out can be used for an approximation of VO2 max.
5k fast is a really good cardio workout.
Also good for intervals if that's your thing.

Well worth having a good look through the Concept2 site, especially the training tutorial (some of the rowing actions you see people doing in gyms are deplorable and downright harmful....), VO2 max estimation, calorie burn estimation sections.

If you learn to use it correctly, you'll get a good workout. When one sees people using it at the gym, they often don't have good technique, and feel that they have to set the damper for max resistance then go up & down the slide like lightning. Not the best workout. If you can't find a truly knowledgeable coach (and some trainers aren't knowledgeable but pretend to be), use the very good info on the Concept 2 web site. They also have a fun online community with challenges, teams, forums, etc. (I'm an on-water rower who owns a C2 and uses it when the river gets crunchy).