What will fit into your lifestyle?
How many times a week, realistically, will you be able to go?
And do you want classes or are you happy pounding away on the machines on your own? If so, is there a decent range of tv channels to watch while you're running/stepping?
I've just cancelled my membership after 10 years.
The classes are aren't mainly at times I can get to (I work FT) so I was only going to 1 yoga class a week, and too busy to go any other time.
Plus, since we've had a dog which needs walking I'm getting plenty of exercise either walking or running with the dog. And we invested in a cross-trainer (bought it off eBay, it's an excellent one), which is in front of digital tv in a spare room.
But when dc were young, and I wasn't a free taxi driver for their own sporting activities, it was ideal because they could go in the free creche/kids' club while I worked out and then we'd all go for a swim or play tennis or squash together.
So, depends what your needs are - try to work that out first and don't tie yourself in for too long so you can cancel if you're not going enough to justify the fee.