Good questions. You are correct that the PPC - vs. - Intel argument has been thoroughly hashed out here before.
I have been needing and pondering a new machine for about 8 or so months now. I would have had one last August (A dual 2.7Ghz PM) had I not moved, the move zapped my savings pretty hard. My current PM G4 is 6 years old and is really not worth upgrading much more than I already have (1.3Ghz G4 upgrade, 1Gb RAM, ATI Radeon 9000/128Mb Video card and so on) as I am a the point of diminishing returns. The 100Mhz system bus is what is really the showstopper these days. I could wait for the Intel PMs that are sure to come in the next 3 to 6 months, but then I would be looking at a Rev A machine, and I really don't want to adopt a brand new architecture on brand new hardware with likely a brand new chip (not the CoreDuo).
I have considered the iMac, but to be honest, I don't really like them. Now don't jump on me for saying that because they are great machines, but they are not what I want out of computer. The lack of slots and the limited upgrade capabilities don't suit what I will likely want to do on the machine for the next few years. Sadly, the Mac Mini and the MBP fit into the same category, although I like them better from an aesthetic point of view (I am considering a purchase of a Mini to act as a Web/E-Mail server though). The upgrade capabilities for Video and the availability of extra slots make the PM the machine that will suit me best. The 1.25Ghz front side bus doesn’t hurt either.
While I understand and accept that the PPC architecture is on the way out, Apple will be supporting the PPC software wise for some years to come, and most software vendors are adhering to the Universal Binaries concept so PPC will have a fairly long life span.
To answer your last question more succinctly, I am getting real tired of seeing the spinning beach ball all the time on my current machine. Yes, I repair permissions religiously and run all of the Cron scripts via Cocktail. I even did a clean install about 3 months ago, but the performance and speed gains were not significant enough to make me want to hold on to this machine as we head towards the release of 10.5. Admittedly, I have between 10 and 15 items in my login items list and these things just eat up processor power and coupled with the 100Mhz bus, it’s just not that pretty. Sure I could remove some or all of them, but then I would be changing the way I choose to use my computer because the hardware just can’t handle what I want to do with it anymore, not something I would relish. After 6 years, I think it’s time for new hardware.
Unlike 6 years ago, I am in the financial position that affords me the ability to purchase a top of the line machine without breaking, or even seriously denting, my budget. Guess I’d like to go for it at least once in my life if nothing else. If the Intel PMs were already on the street or if I were willing to put up with an aging machine for another year, I would buy Intel, but alas, neither of those is really an option for me at this point.
Sorry this is so long, I hope that answers your question...