There are a few people on the Concept 2 rowing forum I use belong to that use these regularly.

Apparently they do work but are really hard. You have different level settings and virtually everyone is on a really low setting and moving up a setting is a big deal.

I've had them recommended to me by people on the rowing forum and my own doctor.

From the sounds of it they definitely work, I think science trials show improvements in rowing time trials of around 2.5% and in running slightly higher.

I guess at competitive levels a 2.5% increase would be fairly significant, certainly the difference between winning and losing. They do sometimes do deals on the older models, which are functionally the same just not quite as stylishly designed.

I remember them doing a powerbreathe classic for £19.99 once on a website so shop around definitely don't pay over $100 for one!