A Canon rep I have talked to a few times says Canon considers the 28-135 to be their "closest thing you can get to L glass in a zoom that's not L glass", and says you can get some pretty good results using one on the 5D.
There are bound to be a ton of folks who are upgrading to the 5D in here from Rebels, 10Ds, and 20Ds, and since the 28-135 is such a popular lens, I'm sure some of you haven't been able to plunk down for some L lense yet and have tried your 28-135s on your new 5Ds. Overall the L series is the best in build quality and usually has superior results, but sometimes a quirky mid-range model comes out that comes close (like the Canon 50mm for under $100) to L quality. If the 28-135 does "nearly" as well on the 5D, it's a viable option to Ls.
Who has done this camera upgrade and is still using the 28-135 to some degree? Any samples you can possibly post?