My Opinion
I bought a 20" G5 iMac last week during Black Friday sale at the Apple Store.
Anyway, I opted for the internal Bluetooth mostly because I have a Palm T5 that I wanted to sync wirelessly. When you buy the internal Bluetooth to get the wireless keyboard and mouse as well.
So far, I'm pretty happy with them. Everything just worked right out of the box. Buy I do have a few observations.
First, after a reboot, it takes a few seconds for the keyboard and mouse to pair with the computer. It can be a little frustrating trying to log on and have both the keyboard and mouse be unresponsive. They do, however, catch up. This also means that you can't use the keyboard to set any of the boot up modes such a setting it up as a firewire drive, or booting from a CD.
Second, they are keyboard and mouse are powered by AA batteries. This isn't so much a problem in the keyboard because 4 AA batteries should last a good long while in a keyboard. But they mouse, being a constantly active optical device, uses only 2 AA batteries which the literature suggests should only last about a month or less under heavy use. (I own a Logitech keyboard and mouse that I used on my PC. One of the reasons I selected the version that I have is that the mouse has a base station that recharges it.)
Finally, I've grown accustomed to some of the niceties of my PC mice. A scroll wheel is a wonderful thing. And, of course, I am a confirmed right clicker.
I hope that gives you some more food for thought when making your decision. All in all, I'd have to say I'm really enjoying my iMac as it seems to be a wonderfully capable machine.