Been using Mother's Back-To-Black as well on the H2 bumpers / trim and, like you said, it doesn't last that long.
Was using the product again the other day, but only had enough left in the bottle for one coat. So I dug through the cleaning supplies and found a full bottle of Meguiar's Gold Class Vinyl/Rubber Protectant that I'd forgot about. Squirted it on and wiped it down and the bumpers still look fresh over a week later...even after several days of rain. Will be using this product now and monitor how it works in the long run. Here's a link:
http://www.meguiars.com/?vinyl-rubbe...ber-Protectant
BTW, for a more permanent solution, you could use something like Griot's Bumper & Trim Reconditioning Dye --
http://www.griotsgarage.com/search.jsp?searchtext=11183. It works great, restores to like new, leaves a satin finish, and lasts a very long time -- may take a couple of applications though depending on how "dry" the bumpers are. I'd recommend using a larger round hand-held sponge applicator though, as the one on the bottle is a bit small. Might need more than one bottle too, just to be safe. I actually used this product to recondition the targa top on a Ferrari 308 QV that I had, and it turned out perfect.
A little off topic, but I also sprayed some citrus-type degreaser from WalMart on the underside of the H2 (sway bar, bellhousing, suspension, etc.) and let it soak in for a few minutes and rinsed. Then, after everything was dry, I sprayed on Griot's Undercarriage Spray on those areas and let it drip dry. It looks like a brand new vehicle now. (see
http://www.griotsgarage.com/search.jsp?searchtext=11138 ).
BTW, I have no affiliation with any of the aforementioned companies, blah, blah...