I have an 8 core, 2008 Mac Pro. I was waffling over whether I needed it or not - considering the price. I like to buy my systems to last 5 years or so, and knew that the amortized cost was not outrageous. And that the Mac Pro would do me for 5 years..... but, it was an expensive machine. Then George Ou at ZDNet did a detailed analysis, and he concluded that at that time the 8 core Mac Pro was the best workstation value out there, hands down. Up until then he generally thought that Macs were overpriced, and he would build his own systems to show how much less he could spend on the same components. Except for the plywood. He built his version of the iMac into a plywood enclosure. Good Heavens, it was ugly! But the actual components were a fraction of Apple's iMac price. So, when he recommended the Mac Pro, I bought mine with no regrets.
Its a year and a bit later.... I have 12 Gigs of RAM now, and I couldn't be happier with a system. Its quiet, fast, stable. Its going to get faster when Snow Leopard comes out. I know that I have no single application that takes advantage of 8 cores, but I can run 8 apps all at once with limited impact on the processing. That Mac Pro is going to get faster still as developers take advantage of Grand Central.
If you already invested in the Apple, and if you have hefty computing needs, then a 2008 Mac Pro is a good deal. If you are looking to switch, then the cost of a Mac Pro on top of the switching costs may be too much. Get a Mini and see if you like the Apple universe. If you do, then you can switch easily. If you don't like the Apple way of doing things, you aren't out too much - and can sell the Mini for a good portion of what you bought it for.
Good Luck.