I have used JB Weld on three rifles; one laminate and two synthetic. It seems to have worked nicely though I have not used any of the other products to compare. One of the members on this site tutored me through the first bedding job and it was not very difficult, with his help. Careful attention is required, as mentioned above, and the odds of a successful job will go way up.
I never did any before and after accuracy comparisons, unfortunately. One job was primarily done out of curiosity as I had always wanted to learn how this was done and the other two jobs were done to reinforce the recoil bearing surfaces (as Confederate Rifleman mentioned) on couple White rifles. The way the stock and action fit together during reassembly, once bedded, is quite remarkable. The tight tolerances and lack of any "slop" in the fit is immediately noticeable.