I have a 1.42GHz DP G4 and I think it is great. It is nice and snappy and does everything I want it to. One of the main plus points is the two optical drive bays, they are a god send.
Not yet, but they are on the official Freescale roadmap. Personally if I had to choose between a single core G5 (and a single processor) and a dual core G4 I would have the dual cores every time. For the stuff I use my Mac for 64-bit processing is not important, I'm unlikely to have more that 4Gb of RAM so two cores would be better.