Should I buy iMac?

[#1]

Quote from: acr

You should be aware that the iMac must be one of the most difficult desktop computers to upgrade or repair on the market.

There's your reason for not buying one!  Form over function...

[#2]

Personally, In 15 years, I have never had a faulty iMac.
Either at home, or doing intensive post production work in TV.

[#3]

and a friend of mine had a MacBookPro go down - graphics problem and the result was that it needed a new main board - or whatever Apple call it - and it was so expensive she scrapped it and bought a Windows laptop which was a lot cheaper and did everything she wanted
This reliability report is worth a read http://ctwatchdog.com/finance/2012-computer-reliability-report-lenovo-most-reliable-acer-least-relia...

[#4]

As a reliability report it seems ironic that their figures aren't very reliable. Well you always need to take statistics with a pinch of salt and look at how they came about. Sample sizes are important for accuracy.
Anyway, hard drive sizes with most Macs aren't that important as you can add more capacity with external drives. It's also possible to start up from an external disk with an alternate system (which means you can run different versions of Mac OSX with ease - ideal if there are compatibility issues with software you've purchased).
Oh and Jim they are call 'logic boards' 

[#5]

Quote from: adagio

I have been a Windows user since Windows 2 (yes I am old) but have been captivated by the 27" iMac that I have seen and played with in John Lewis. It is an object of beauty with a stunning display. My existing, XP, machine is getting rather long in the tooth and needs  replacing. Should I buy the iMac?

If the hardware appeals to you, go for it.
I've been running GNU/Linux on a MacBook for a few years now; installation was no different than any other x86_64 hardware I've used.
You might find you can install Windows 7 / 8 on the iMac if you'd prefer to stick with Windows