I picked up a decent used Minoura trainer in the fall of 2011 for a good price, and I've been happy with it. Trainers are a great way get some good saddle time in these colder winter months (though I do get outside when I can with the appropriate cold weather gear). The biggest problem with trainers is that they can be very boring.
At first I simply listened to music while I rode, but that got old fast. I used my trainer maybe a dozen times over the course of last winter because I just couldn't bring myself to ride it. This winter season, I was determined to ride more. I started off watching YouTube cycling videos (on-board rides and Tour de France stages); that was alright, but still not great. But then I found some cycling workout videos that are used in conjunction with trainer workouts--now I look forward to riding my trainer!
The videos I use are from Ride-Fit
(I bought their California package). The videos were shot with on-board cameras mounted on the handle bars. Throughout the video, you adjust your resistance settings and cadence in order to simulate a real ride. In addition to the video, I also listen to music. Since I started using these videos, I've been putting in a lot more time on my trainer. And as I said earlier, I actually enjoy it now. Is it as fun as riding outside? No way. But it sure does beat sitting on the trainer and spinning. The rides are challenging and the hour+ goes by very quickly.
To sum it up, a trainer is worth it if you actually use it. I suspect that a lot of people buy trainers and rarely use them because of they're not the most exciting thing in the world. But, at the end of the day, they're a great way to keep your rear end on your saddle on those days when getting outside just isn't feasible. The trick is figuring out how to enjoy the "ride."