I drive a 4.8 Yukon and I love it. When I first bought it, I would notice it lost a little bit of speed on steep inclines and the engine would have to downshift...but like I said that was only on big hills. Since I've bought it I installed a flowmaster force II exhaust system and a K&N 77 series metal intake kit and that issue is history. The exhaust alone made a little difference but the COI really changes things. On the 4.8L K&N guarantees a 14.5HP gain. The extra power in acceleration and cruising speeds goes a long way...not to mention it sounds great. W/ these addons, you're running more HP than the 5.3L comes with stock anyway and it only cost me about $400.
All in all, I'm very pleased with my 4.8. If you're a guy like me that never keeps your truck stock for very long, its a great buy. Besides, the 5.3L only has @ 10 more HP anyway from my understanding.