I've been using a Mac G5 desktop and a G4 Aluminum PowerBook for the past 5 years. Each unit cost around $2500. Ouch. Each unit has broken down once. The G5 had a processor replaced. The PowerBook's logic board started acting up and then bit the dust.
The repair experience was excellent. I'd rather they not break, but stuff happens.
The G5 is still holding its own in Logic with two 2Ghz processors. The thing is still ferocious even as it gets a bit long in the tooth. The PowerBook can run Logic and MainStage enough to get through a live set, but anything else will kill it, so I'm considering a newer model.
I made a trip to the apple store today and loaded up a benchmark file that runs 100 tracks EXS24 with a ringmod, reverb and some other effects on it. You can unmute the tracks until logic chokes.