I've been using a pair of cheap BB frames and photochromic lenses from optilabs.co.uk for the past 4 years. Gone through 2 sets of frames (they're cheap at £22) and 2 pairs of lenses (due to scratching them) in that time.
They work pretty well and are reasonably priced.
Have been thinking of some Oakley Flak Jackets with prescription photochromic this summer but at 300 quid I'd be too scared to take them mtb'ing!