I'm the guy who originally asked the question...
Your three points: quality, cost, and 'fun'
I can't really say too much about the quality myself, but I'm guessing that they dont like you overclocking your parts, and that should void the warranty, no? You say I could get 'exactly the system that I want', but the only other thing missing from this system (for my purposes) is a SSD boot drive, which I could easily add and use for that purpose.
Cost: Over here in the UK, a HD5870 goes for £300, a 40GB Intel X-25M Gen2 SSD goes for £100, and the RAM goes for £200. So, that's £600. This PC here costs £800. So I have £200 to spend on my case, my processor, mobo, power supply etc. This system, to me, offers exceptional value for money. (The prices stated were the cheapest on google shopping, by the way).
Expandability - if I was going to build my own PC, I would roughly follow the enthusiast guide on this website. They recommend the same socket. I would not really consider buying more than one graphics card, but if I did I would just use the weaker one on one of the *gasp* smaller empty PCI sockets there are! Sure it might limit the frame rate just a teeny tiny bit, but then looking back at the cost advantage to me, it is worth it.
Fun - sure, if you think so. This is, obviously, the most personal of the things you said, so in my opinion it doesnt really matter too much.
Sorry if I offended you somehow, but this PC is probably a better deal in the UK than the US I guess... I was trying to find out if there was a huge advantage to building your own that I had completely missed...
PS: Sorry if some of the sentences are badly phrased - cant be bothered to proofread it.