It's just like every other trainer out there (other than rollers) in that the rear wheel is fixed.
I originally bought my Omnium to travel with and to warm up at the occasional TT. It's a great piece of kit as it folds up pretty small, is relatively light, and is easy to set up and take down. I've been traveling a crazy amount this year for work and have taken my Omnium and my race bike with me accordingly.
The resistance of the Omnium, for me, is plenty for sustained efforts. For sprinters putting down big wattages it might not be workable. The biggest feature, though, is that the fork is fixed and the rear wheel is free to move side to side a bit. For me this makes a huge difference in comfort on the bike during training sessions. Also, I've rediscovered the art of pacing. Because I'm not in erg mode constantly, I actually have to be conscious of my effort. One thing I've noticed while spending time over on the timetrial.co.uk forums is how few people use erg mode. Yes, erg mode is a nice luxury when you need to bang out a three hour ride at a steady effort... which can be mind-numbing. That said, I think there's a lot of value in doing a 2x20 on a "dumb" trainer or in "slope" mode so that you have to focus on your effort rather than just pushing the pedals.
Just some random thoughts. Feel free to ask a question or comment.