Phil/Chris-I have both (bbb's and MT500).
They will both fit over MTB style shoes. I have
Shimano MT22 shoes .
I bought the BBB's online around October 2010. They got me through last winter and up to October this year. I used to walk in them to/from the garage, to/from cafes, to/from the underground carpark to the office at work. With the shoes I have the front of the overshoe sits quite high as my shoes are rather flat and I think they are designed for generally larger type MTB shoes.The piping around the 2 holes on the sole then started to fray and come a little loose.
However, they suddenly started letting a lot of water in-a lot of ingress from underneath. They are now a little loose fitting, in particular the holes underneath sit proud of the sole of the shoe. I think the constant stretching over the shoes has caused it, coupled with the fact that the front of the overshoe was too big for my shoes in the first place..
I then bought the MT500 overshoes. The fabric is tougher and they fit more snugly. I continued to walk in them as I did with the BBB's. However, within 2 weeks of buying them the piping around the holes on the bottom had started to fray, pretty badly too. The issue is that the piping is stitched on with what looks like weak thread (yellow in colour). I still use them as they are still waterproof, but now try and remove them/put them on as soon as i step off/on the bike.
All in all I would go for the BBB's (given my time again I wouldn't buy the MT500's), but making sure that you buy the size such that the overshoe fits snugly to the bottom of the MTB shoe.
If you like you could try my BBB's giving me a fair price for them?