Considering that I upgraded from a 1.3Ghz G4, this is like night and day. The G4 had a 100Mhz bus, which what ultimately became the bottleneck, and couldn't really take advantage of new, fast drives. This thing has the 1.25Ghz bus and utilizes only SATA drives, no IDE interface built in.
So far I have no complaints, it's nice to have Apps open in one or less than once bounce and to have things like iTunes and iPhoto open fast (they both took forever on the G4, mainly due tot he size of their respective libraries). Have not had any problems with the hardware. Bought it with 1Gb of RAM and then immediately added a 2nd. All can really say is FAST, and I mean FAST.
The one area I have used where performance has really shown, and which utilizes all 4 processors intensively, has been ripping DVDs for my iPod. The G4 took 4 to 8 hours for a regular DVD, H.264, Single pass. My ibook takes 2 - 4 hours. The Quad takes about 25 min or so. That is what I am talking about.
If you have any more specific questions. let me know.