Had it on my 5D Mk1 for a while - I was very impressed. Replaced it with a 24-105 L, but the 28-135 seemed sharper to my eye. Only after AF microadjustment on my 5D Mark II has the 24-105 become a really sharp lens.
Go for it - you won't be disappointed. Excellent value, great range on full frame, and good build quality (and fairly light weight).