Hi Wolfeye
Welcome to the Pentax 645 club!
I picked one up on an auction site myself a couple of months ago, and since then I've been having a blast. Should have got one years ago!
I echo all that has been said thus far - about both the good and bad points. For slide film I too wish it had half or 3rd stop adjustment and aperture clicks, but if I bracket a lot there is always one (or two) in the series that are on the money.
I actually find the push button interface really intuitive - perhaps my background in push-button slr's helps (my very first camera was the Canon T70 - very similar - and at the time quite revolutionary - push-button control).
I toyed briefly with the Pentax 6x7 as well (borrowed a friends), but it was 'too much' camera for me. Too big, too heavy, too clunky...
The 645 image area seems to be about right, and is definitely a step up from 35mm (and digital with a good scanner).
Perhaps the thing I like most about the 645 (and indeed other medium format film cameras), is the way it makes me slow down when I use it, and really appreciate the 'art' of photography again. Even though I can, and do, hand-hold it, it certainly doesn't promote a 'shoot-from-the-hip' approach to photography in the way that my 30D outfit does.
I have written more on my experiences recently on my blog
www.nzdigital.blogspot.com feel free to take a look.
Congrats on the purchase. It's a great system, and a lot of fun.