I run spin classes, we have a lot of different spin trainers in use at them, and many riders use magnetic trainers and wind trainers, some use fluid trainers.  What's the difference in general? See here.

This isn't a comparo of mag vs fluid vs wind, the jury's been back a long time, and the superior technology is fluid.  I have two fluid trainers myself, a Cyclops "Fluid 2" and a new Kurt kinetic "Road Machine".   I'm very impressed with the Kurt Kinetic trainer.

What makes it so impressive?  It's not cost, it costs, in Australia, around the same as the Cyclops Fluid 2 (between $500 and $550 or so, depending on the bike shop) and uses comparable technology (impellor in a fluid bath).  They both have effectively infinate exponential resistance curves.  They're both good for sprint training as well as endurance work, noting that mag trainers are not good for sprint work and wind trainers have a habit of breaking at high loads (not to mention sounding like a jet engine at high rpm's!).

It's a couple of things.  Firstly, the flywheel is heavy.  Really heavy.  Much heavier than the Fluid 2.  Why does this make a difference?  More realistic resistance.  Spin trainers give you constant resistance through the pedal stroke.  This means that, undamped by a heavy flywheel, you have to work harder at the top and bottom of the pedal stroke on a trainer than on the road or track.  On the road and track, your own inertia carries you over the dead spots, on a trainer, no such luck.  This leads to a (maybe desirable, but unrealistic) extra load on muscles that don't generally have that much say in just how hard you're working, leading to earlier and unrealistic fatigue and the potential for a less effective training session.  A big, heavy flywheel reduces this problem, and the Road Machine has a heavy flywheel.  You can see this in action, spin the trainer by hand and watch how long it takes to coast to a stop.   You can really feel this in use too.  It gives a much more realistic road feel than the Fluid 2 does, and outguns the mag trainers as expected.  For the rest of this review, I won't mention mag trainers.

The Cyclops has a moderatly heavy flywheel, but it's not even close to the quality of the feel from the Road Machine.

What else?  Accurate power calibration.  Using a trainer is great, knowing how hard you're working is very valuable.  Unless you have a Power Tap or SRM cranks (or the Polar 'wild guess' voodoo power meter) you can't really tell how hard you're going.  You can use a heartrate monitor, and that's good, but it doesn't give you watts.  Watts are good.  Watts are what it's all about.  The Kinetic has been calibrated against a Power Tap and has an optional computer that tells you your wattage.  Good?  You bet!  It won't tell you torque or peak power used for sprints (for that you need a Powertap or SRM) but if you're doing endurance work this is great.  The Fluid 2 has a noticable change in resistance after a few moments as the fluid in it changes viscosity with temperature, but the Kurt Kinetic uses a different fluid that is much more stable across the sorts of temperatures that trainers work at.  Of course you can get the same from any trainer if you use a power meter of some form, and for sprint work it's not very useful as a meter, but it is great for measured endurance intervals and for sprinters it does provide more than sufficient resistance (just like the Fluid 2) to be a useful training tool.

A bigger roller?  Ok .. hard to notice, but the trainer is noticeably quieter than my Fluid 2.

Some internal niceties, the Kurt uses a magnetic couple to link the roller and flywheel to the impellor, so there's no chance the thing can leak fluid.  Early Fluid 2's were a little famous for this, but not in my experience, but it does seem like a better idea than a shaft and seals.

Some weaknesses?  Yep.  It's quite heavy (as you'd expect, that flywheel ...) so transporting it is a bit harder - I used to strap my Fluid 2 to a backpack and ride to spin classes, but I'd not do that with a Kurt.  I'm not thrilled about its behaviour when doing standing start high gear efforts, it seems to lurch for the first few pedal strokes before settling down once you get your cadence going.  This isn't a common use for a trainer, but that's what it does.

As someone owning both a Fluid 2 and a Road Machine, I know which I'd rather use myself, the Kurt Kinetic is a winner.