Is it worth buying a really cheap mountain bike?

[Magog]

I've missed cycling for far too long, been out recently on bikes borrowed from friends and really enjoyed it again. When I was younger I had a couple of specialized rockhoppers that I really liked (but both ended up stolen, bds), I can't really afford to buy anything like that now, but I'd really like to get back in the saddle and get a bit fitter.

I'll probable mostly be riding on road and towpath type environments to start off with, so nothing too arduous. My question is would a really cheap mountain bike from somewhere like Halfords or Decathlon for around 100 pounds be worth it, or will it just leave me severely disappointed and put me off cycling forever.

If I really needed to I could probably stretch my budget up to 250-300 pounds, but Id then be planning to keep the bike a lot longer, and it's more money than I can really afford at the moment. I'd rather save for a bit and get a more expensive bike later on.

I also don't really want to buy secondhand as having been the victim of bike thieves previously I'm rightly or wrongly very wary about gumtree/ebay etc.