I agree, the full frame sensor has great advantages – the most important it captures more light. This reduces immensely the noise of the digital picture. If you are in available light photography, which i do a lot, efficient use of the light available is crucial.
There are some disadvantages, too: The price of the body is simply horrifying, and the lenses, too, are very expensive. The lenses are bigger and heavier, a problem especially with telefoto lenses.
For me, Canon produced with its EF-S series a serie of great lenses with really high quality, but only to use with the smaller sensor.
Especially the 10-22 (translates to 16-35 for the 35mm (full format) sensor) and the 17-55/2.8 (translates to 27-135 mm) are wonderful lenses which i use all the time.
Therefore i now changed from the Rebel XT to the wonderful new EOS 40D instead of waiting for the successor of the 5D.
Nevertheless i am saving for some FF-Lenses, especially the 100-400mm IS.
I agree with you lechuck, it is all about the lens you have – think of it: you will buy a new body every few years, but you will stick to your lenses!