I agree that the original effects were not going to fly in HD. The live-action footage looks great, but the effects would be jarring to anyone not used to them. The same washed-out fly-by with half the nacelles missing was reused again and again, along with that exact same 'turning shot'. I really think that the effects are really supposed to suppliment the action, not take away from it. So I'm cool with redoing them in HD.
Having said that, the CGI has been a bit underwhelming at times, and the colors were screwed up a lot in the beginning. Not just the horrible looking fruit-loop nacelle caps, but the hull color. The deflector area seemed to wash out into all-grey for a while. Finally, the blacks were not really that, well, black. Maybe it's just the screencaps, but if you look at the comparison shots over at Trekmovie, the original shots have black shadows. The new shots have grey shadows and space itself looks grey!