I've been using the Cateye Rapid 3 Auto tail light (TL-AU630-R) for a while but I've just placed an order for the manual Rapid 3 (TL-LD630-R).
I don't usually ride at night so my lights are just for occasional use when I can't make it home before dark and when I want to be more visible before dark. With the Rapid 3 Auto I can't make it stay on until it's almost dark so I think I'll be happier with the manual kind.

I've tried partially covering the light sensor in the Rapid 3 Auto with a piece of tape but it's difficult to do this without covering too much of the LED's at the same time.