A few months ago i replaced the glazing rubber in the outside of our conservatory, as the old stuff ahd shrunk and lifted and split. I ordered a sample of the various sizes of rubber from trademe which was around $9, and a glazing tool to fit the rubber (which is worth its weight in gold). After working out which rubber was the best fit, i ordered a roll of it and spent about 2 hours replacing the rubber. Easy as to do once i had the glazing tool and the correct rubber, even for someone as useless as myself with diy. Certainly made the conservatory glass non leaking when it rained.
There is a nack to it, but its not really all that tricky. It is however rather time consuming which means paying a company to do it makes it a really expensive job (when the rubbers is only about $1 per meter)