Auto-cleaning is absolutely worth having. It works, and works brilliantly.
I own or have owned 400D, two 40Ds, 50D and 1D III, and have
never had to manually clean te sensor on any of them, while my two 20Ds (which don't have self-cleaning) are constant dust bunny warrens. I would not even consider buying a camera that didn't have this feature. (Bear it in mind that I work exclusively outdoors, and mostly in a hot, dusty environment.)
I don't thinks auto sensor cleaning in cameras is that effective. After all, what does it do? Just vibrates the dust off the sensor. And what happens to this dust? It just goes back onto that sensor - it isn't removed from inside the camara. Eventually your cameras' insides will look like the inside of a hoover bag!
Not so. If you read the literature that the manufacturers provide, you will discover that part of the self-cleaning package is a dust-collection area - it amounts essentially to something like double-sided sticky tape, and this is where the dust ends up eventually. If you hold the camera horizontal while it's cleaning itself, the dust drops straight down onto the pad. If you hold it at other angles, it floats around for a while before some random motion places it in contact with the pad, but either way it gets there in the end.
Naturally, you help it out by blowing any surplus dust out of the mirror chamber with a rocket blower now and again, and any time you send your camera in for service, Canon replace the pad with a fresh one as a courtesy.
Finally, note that the dust cleaning system is just that: a
dust cleaning system. It cannot do anything about smuts, grease, oil, or slug trails! But if
dust is a problem for you, the self-cleaning sensors are brilliant.
PS: David, the 1D III is simply superb. Scratch up some currency - sell your grandmother if you have to - and go for it. I still use my various other bodies for some stuff, but the 1D III is by far the best of them. Shockingly expensive, but worth it for sure.