I think, for most folk, there is no need to upgrade to vista at the moment, or anything else, unless they succumb to the marketeers. Gradually, as they buy new hardware (printers, scanners, and so on) the earlier drivers will not be included. If they buy a pre-built machine, it may come with vista pre-installed. They can put on whatever os they like. The PC is an open architecture. Windows is probably the most used pc operating system, thanks to good/bad marketing deals by M$. (I changed 'most popular' to 'probably the most used' because I have no idea on the % of linux/other os installations, and every heavy user of windows loves to hate it)
However, my current pc, running windows xp, will run software I wrote in 1980, before windows was even around. M$ are now generous enough, that they give free development software. I am able to write stuff, without to much difficulty to control a machine tool, say, something of my own design, via a serial interface, from a local pc, or from one of my network pcs or even across the web. The cost of the development software - nil. If I want a full 'enterprise development version' I can pay a few hundred pounds and get that.
I can purchase first rate software - qimage, and so on, at almost give-away prices, and there is plenty of first rate free-ware there too - Irfanview, Picasa, etc. Some of the more technical stuff I use will never be available for the Mac, the market is just too small among the technical/engineering users.
There is no need to upgrade to vista yet, I expect the raft of add-ons I have may not have drivers now, but in a year or so time, it may be worthwhile. Most of the teething bugs will be known by then.
I see no advantage for me, in any shape or form, to go to a mac type platform, but mac is coming the windows way.
It could be worse, we could have had Adobe trying to write operating systems... I just do not understand their mindset at all, maybe its they show the trait of a wannabe, but are just not being quite good enough to make it.
Best wishes,
Ray