I have lost the tripod mounting ring for my Canon 70-200mm f/2.8 IS USM. It is very expensive to replace it with an original one [about £140.00 as of May 2012]. I have noticed that there exist compatible third-party rings. Will they hold my very heavy lens securely on a tripod?
Tough to answer authoritatively because of the wide range of quality from various vendors. I can only give my own experience: I've been using a third-party (eBay/no-name) tripod ring for my 70-200 f/2.8 (NON-IS) for a number of years (I believe this is size 'B' which is slightly smaller than what's needed for the 70-200 2.8 IS).
It seems reasonably sturdy and I've never doubted its ability to hold my lens securely. I have not noticed any externally-visible defects (cracks, etc.), though I do inspect it each time before using, just in case.
One thing it lacks that the original Canon rings include is the alignment markings, which is occasionally an annoyance. The fit was also a tiny bit loose, which I supplemented with some stick-on felt on the inside of the ring. These will probably vary from vendor to vendor.
You may want to consider the answers to this similar question: