I am kind of relieved that I'm not considering an upgrade on this cycle for all the following reasons...
It's a REV A, the smart money is to generally avoid REV A's as they often have "issues" which get ironed out on the next revision. The whole REV A thing is compounded by the fact that they have obviously had manufacturing problems and are perfecting new techniques: Firstly we heard it was laminating the screen to the LCD array and now (if this rumour is to be believed) the actual welding of the chassis. Also, it will be interesting to see how the new arrangement of parts works especially regarding heat dissipation - many people are rightly excited about the 680MX GPU upgrade, especially with a view to gaming, however driving that card hard is going to skyrocket temperatures in that enclosed chassis.
The other issue I have is I can see a far shorter life cycle for this iMac, The design is now in the bag but Apple will be keen to tweak the design / engineering / Production process relatively quickly to resolve any real world issues the design throws up. With Haswell being available in around six months time they'll likely chuck those in with the possibility of the latest GPU and, my guess, better storage configurations - I'd guess by late 3rd quarter early 4th quarter 2013 (it's like the iPad 3 all over again😉)
It's a tricky one for those who have been waiting for this iMac release and if you need the computer then you just have to go for it - let's face it when that new shiny machine first gets switched on in your house or work we all know that feeling.