I would say it depends a bit what you're going to watch. My 4280XD is fantastic for HD and SD TV, via Sky HD - I've yet to see a more natural picture. I watch loads of sport and it's great for that. BBC dramas still in SD, like Spooks, look very good indeed.
The Kuro upscales DVD (I send them at their native resolution) very well. This is important because I'm interested in the quality of the film above all!
I send a 1080p signal from blu-ray and the PQ has many virtues - colours, black levels, very good definition. In many respects, it's still the best. However good the picture might be, it isn't quite as sharp as a full HD set, though. Overall, it's really excellent (Avatar looks spectacular) but edges are just a little soft.
So there you are - the Kuro suits my present viewing admirably. It's a wonderful TV and I won't be changing it until I'm fully convinced by a new generation full HD set. You'll have to decide whether it fits in with your needs too!