It can be useful to a lot of people, although I do think it should be viewed with a long-term approach rather than something you are just going to run a few bottles of. I've been using a shellfish-free Glucosamine for a number of years now purely as a preventative; not sure that I notice anything directly from it but considering the amount of running I do my knees, hips and joints in general are ache and pain free. I do feel that it is helping, even if that is simply due to helping the preservation of cartilage.
One thing to bear in mind is that it is relatively slow to start working (general consensus is anywhere from 4-8 weeks); synergistically it pairs really well with a good quality Omega 3 as this will provide a certain degree of anti-inflammatory benefits. Dosage wise I would be aiming for at least 1,000mg/day but ideally probably closer to 2,000mg.
Just my 2c, cheers MF.