I am a member of a gym and was going regularly until I lost my gym buddy. It's no real excuse I know.

They can be great and give you a good variety of workouts but only if you attend. I wish I had gone casually to get a feel for it before locking myself into a contract. I find it much easier to motivate myself to go for a ride/walk/run from home than to have to drive to another location to do it.