I have a quad G5 in my home office and still use it on a daily basis as a desktop system.
The system is still fast enough for most tasks. Compared to my 10 months old Macbook Pro 17" it is still about 50%-100% faster, depending on what you are doing. So still pretty fast.
I don't do video a lot, so rendering time is no big deal for me. I am using it mainly for my music studio, office work and graphics/photo/layout. Despite Apple's statement, that Logic 9 runs only on Intel machines, it is a UB app and works great on the quad, performance is fine... But it seems that Updates are only for Intel machines. have to investigate that still.
What sucks is, that PPC support is dropped everywhere for Apple products now. I would love to use e.g. Snow Leopard on the Quad. I also need virtual machines for my work.
So sometime this year I will switch to a Mac Pro, which is a pity, since performance wise the quad is still completely sufficient and a very reliable machine.
groovebuster