I have a Trek 3900 that I got this past spring. It's a sort of mid-range mountain bike. I ride it a lot, when the weather is fair...it's probably the most fun I've ever had, and it's excellent exercise. I've got a bike rack that mounts to the spare tire on the back of my rig for easy hauling.
I want to do more riding next year. My goal is to be able to ride over one of two mountains in the area (Stone Johnny or Baldy Mtn) by next fall. Trails range from easy to difficult in most areas, so it's not like you're instantly thrust in to the world of ten foot drops and giant boulders.
I'd say I'm almost intermediate by now.
It's important to know what you're looking for in a bike if you're thinking about getting one. A road bike has a light frame and thin tires, so it won't really be suitable for trails. A mountain bike has a heavier frame and thicker tires, and while you -can- ride them on pavement, it's not the best experience.
You also have to consider what sort of suspension you want. Mine only has front suspension.
The list goes on.
For a mountain bike, you can either go to walmart and buy a $50 cheepy to see if you like it, or you can go to an actual bike shop and spend about $200-$400 on a nicer starting bike. The difference is in manufacture and quality. For the walmart bike you get something made in China from a store that can't really answer any questions or do any service on it. For the higher price you get something durable made in the U.S. from a knowledgeable source. Everyone I've talked to says it's been worth it to them and so far I have to agree.
The other upside to spending more is that it retains some value. A year or two later you might decide to upgrade to a nicer bike. If you bought the Chinese bike, you can sell it for maybe $5-$10. If you bought a U.S. made one, a $200 bike will still have $75-$100ish value as a used item depending on condition. For someone else getting in to the sport, it's still a good way to try it out. Or it's also a way to get someone to come and try it with you if you keep it and have a spare bike laying around.
They do come with maintenance costs. You want them serviced at least once or twice a year (not really expensive and usually your first one is free). There's tools you'll want to slowly acquire to be able to do certain things in the field, spare inner tube etc. especially if you get a mountain bike. And you'll want some type of hydration system, probably. A water bottle might work fine for you, but if you really start getting in to it you'll want to look at hydration packs like the CamelBak, which starts around $50 and also has a couple of storage pockets for energy bars and bandaids and the like. I got one and it's completely worth it...but then again I also use it for work sometimes.
Anyway. To be brief, yes I have one and I use it lots when weather permits and there's not snow and ice and slush everywhere