I've used spin bikes, lifecycles and my bike on my trainer. I think there are advantages to each.
Spin:
Good -- Group atmosphere is very inspiring. If I am almost ready to drop, but I've got 5 more seconds to sprint, and the class is sprinting and the music is screaming, I keep sprinting for those 5 more seconds.
Bad -- The bikes do not fit me well. I have seen people bring their own saddles and pedals, but I'm just not into that. Also, some spin instructors are better than others. The concept of pedaling with the music and your foot should be at 6:00 on the down beat encourages mashing, not spinning circles.
Lifecycle:
Good -- Preset program makes you ride a certain resistance until that segment is over. You can't cheat (unless you push some buttons).
Bad -- Bikes have VERY little adjustments. Saddle up and down, forward and back. Most do not have foot straps, so you will NEVER be able to ride one with good pedaling form.
My bike on my trainer:
Good -- Exact fit! Convenient for me (don't have to go to the gym or be on time for a class). I use Spinervals DVDs with my own music for motivation.
Bad -- Expense. Cost of the trainer, Spinervals DVDs, tire use. My trainer EATS through tires fast! The tire gets a big flat spot in just a month or so. So I bought an extra rear wheel and I leave an old, dead tire on it.