I'd get it if I could afford it.
The high end G5 is probably the best computer you can get in PC-range. It's powerfull and uses all high-quality hardware. Not only you get the dual power970's, but you get independant bus for each proccessor and 2 independant pipes to the RAM. Even though you could duplicate the setup sortoff with "wintel" hardware, you couldn't come close to using the potential of the hardware that is provided by the overall motherboard/architecture design, since you will be working with a archacture that hasn't realy changed much since the 486's, versus a PowerMac G5 that has the advantage of being completelty new as Apple needed it.
So it would be fantastic for stuff like Video editing and multimedia stuff. You could say that the latest Intel is more powerfull then a single G5, but it is fundamentally limited by the motherboard bus.
Example would be "live" video editing at high quality. Using a IDE HD you have moments were certian ways to create/copy/move files can use 90% of a very fast proccessor. A Intel can handle that well, but even with the quad speed bus you are going to be losing a lot of CPU potential simply because the rest of the computer can't keep up. So that you are going to strangle the proccessor and it won't be able to use it to well to do the actual editing/encoding or whatever. Even with dual proccessors your still having both CPUS fighting over the same bus bandwidth, making the problem even worse.
A dual G5 however has all the bandwith aviable all the time for both CPU's. So even with one cpu going all out moving files and stuff, you still have the other CPU aviable at it's full potential to do the encoding, editing or whatever.
Plus you get the nice stuff like PCI-X slots for more bandwidth to them and the SATA HD.
It's all about thoruogh put and getting the most out of the aviable componants.
For example even the old 500 mhz G4's are still usefull and with enough RAM can run OSX well plus all the latest 2-d image manipulation programs without any issues.
That being said.
The new Opterons are close to being that efficant and the faster CPU speeds will make them more powerfull then the MACs. With the highest speed busses, Intels can hand with the G5's with out much problems. The Intaniums are probably faster, as will the IA-64's comming out.
Plus they are usually cheaper. You still have to deal with the x86's traditional weirdness, but most people are used to it by now.
One of the major advantages (to me) is that I like Mac's, but I like Linux more. However their is a nice programs that will enable you to run OS X inside Linux on PowerPC platforms. This is with little slowdown at all vs running OS X by itself (as long as you have a lot of memory). SO you get both platforms able to run at the same time with good performance. You kind of are able to have your cake and eat it too.
however I doubt I would considure buying anything other then the highest end dual G5. The others G5 stuff doesn't realy have anything more then you can get thru the x86 platform (and at a cheaper price).
to see were I am coming from check this out