Lodger, as I understand it MSM is important
in helping the body replace joint tissue, such as tendons and ligament. Its an active source of sulphur which has an important role in such activities.
Hyaluronic acid is not very well known but I learned about it in a US running magazine. Its a natural polymer which helps regenerate cartilage, hence its specifically for joints. Its not cheap, I think about a tenner for 30 tablets, one a day.
I get all of my stuff from Zipvit.co.uk
I agree with a lot of the sentiments above, its pseudo science and not well researched, but most of the stuff I take has some backup evidence to suggest that they aid joint mobility, reduce osteoporosis and generall protect against wear and tear.