My new iMac 20" has finally arrived and after a bit of data transfer time I can finally give a bit of an off the cuff review.

System : iMac 2.0GHz duo core. 256MB video ram. 1GB RAM. Everything else is standard.

Build Quality - fantastic. Feels solid, quiet like a mouse, everything feels top notch. Good keyboard.

Speed - fast. Really, really fast. Applications that use Rosetta are noticably slower; a big exception is Skype - it takes both processors up to 95%+ while it is running. I probably will be avoiding Skype until the Universal Binary version comes out. Diablo2 and Warcraft 3 run nicely though! Overall the system is probably not 2x faster than the iMac G5 but waaaaaay faster than my old iBook, which is all that really matters.

Screen - the giant screen is just a wonder to behold. Hugely better looking than my old iBook's screen. Good colour, brightness, and sharp. No dead/stuck pixels; just plain gorgeous.

OS X 10.4 (Tiger) - nice upgrade. not a huge amount new here, lots of under the hood tweaks, connecting to my WPA secured network was very simple. A lot of the "what port/protocol/etc are you using" style questions have been replaced by what I would assume is a scanning scheme. Spotlight is nice, works well. Dashboard is a bit of a "meh" but I will play with it more. Auto-detected my UPS and I can configure this in the Energy Saver panel.

Built-in iSight - uses USB 2.0 internally, good quality, again a "something I might use"

Front Row - kinda neat. Would be very interesting if it integrated with some for of TV interface/listings/PVR functionality.

Mighty Mouse - or the mouse that I am not nuts about. It's a decent mouse, I like the overall size and such.
Problems are:
- the squeeze buttons are not very tactile and I have difficulty pressing them
- the right mouse click only works if you lift the left mouse click finger
- the middle mouse button is a bit quirky

Overall rating : 9/10
Definitely be wary of non-Universal Binaries. The use more RAM, are not as responsive (e.g. Skype), and use a lot more CPU time (e.g. Skype). I would highly recommend this system unless you figure you can hold out for the next generation of iMac.
And I would recommend getting a different mouse, but your results may vary.