The issue with trainers has been their infernal noisiness. Sure, some of the features are designed to give a better workout -- making resistance resemble riding an actual bike in the real world -- but anybody who can reduce the noise has a good shot at selling some trainers. The magnetic trainers were a big improvement over wind trainers. But, the fluid trainers at last are just about as quiet as plain rollers -- that is, not very quiet, but possible to hear a radio or TV over.
The Kurt Kinetic or Cycle Ops seem to be the good ones (though I've never actually seen any of the "bad" ones that supposedly will leak oil).
Rollers are great, too. When riding, you have at least three cylinders rotating at high speed, so there has to be some noise. But, the good ones (Kreitler; but even lesser brands seem to have better bearings these days) are very free of vibrations. The resistance isn't usually as high as the fluid trainers. The big claim for rollers is that you improve your riding skills at the same time you work on strength. No doubt true to some extent. Weigh against that the need to be constantly vigilant so you don't ride off into your living room -- personally I like to be able to space out without penalty.
But, the short answer: I wouldn't use a mag trainer if I could help it. Certainly I wouldn't buy one these days.
