I hired a bike last year for £50 and it was great. Get there as early as you can in the term because you'll have the biggest choice of bikes and they usually run out. My bike was great - brand new, value £350.
The best thing about it is that the Bike Hub is open throughout the term. The team there is really friendly and you can take your bike down there if you need to do any repairs, cleaning etc. They also offer proficiency courses if you've not been on the road for a while. Only downside is that the opening times are a bit inconvenient: Tue-Thu 12-4 (unless they've changed), but you get a hell of a lot for your £50.