I guess that it really comes down to a question of whether you can find something there that you enjoy enough to go in multiple times per week, and it also depends on what your local bike trails are like and what the local climate is like.

I started going this past winter, and I have to admit that there are some things there that I have kind of gotten into - enough that I go in 3 times a week at about 5:30AM. But my experience was that if I weren't doing this, I would only do occasional rides on a trail somewhere, and that just wasn't enough. I guess the other part of it is that at the gym I don't need to worry about weather, bonking, traffic, being under/over dressed, flat tires or much of anything else.

Lately the thing that I have kind of gotten into is a machine that is sort of like a video game where you are racing on a mountain bike. You have a video screen with a course, and you have to beat 5 other virtual riders around the course. There are multiple courses to ride in different virtual climates - some have climbing, some are straight, some have lots of twists and turns. If I were out on my own, I would just be taking my time - the element of competition (as artificial as it is) gets my competitive juices flowing and I ride harder than I would otherwise. And after I get done with this thing, I get in the pool to swim laps (and cool off from the ride at the same time). And after that, 10 minutes in the hot tub .