I use my trainer in the winter months when it's super cold and icy on the roads and too dangerous to ride. I think you definitely do get "used" to your position on the bike when it's on the trainer - esp if you're doing hours long training sessions on it. I usually put my tri bike on the trainer so I can get used to being in the aero position for hours throughout the winter. I always feel like I get a great workout too on the trainer - it's harder for me to keep my heart rate up on the trainer than outside riding. I'm not sure why, but keeping it at around 150-160 bpm on the trainer is killer, while I really have to slow down and pay attention to keep it that low while riding outside (that's the aerobic range I keep in doing base mileage in winter months). It's a different feel but still a great workout - an hour on the trainer feels like 2-3 hours on the road!

But whatever trainer you do get, be sure to put a cheaper tire on your rear wheel so it doesn't eat up your $$ tires. I have a cycleops fluid trainer, and it still shreds my tires slightly, so I switch out to a $5 tire I bought at performance bike a few years ago.

Good luck and have fun!