Whilst I am far from a PC building expert, I have built several over the years. I am quite into recording music, games, high definition video etc and I am a self confessed gadget freak. My last PC which I built myself cost me ~£1000 to build, it was at the time a very high spec computer. When looking around to build a new one, I found it was actually cheaper to have it built for me. That said, the arse has fallen out of the custom PC building market. There is probably ONE good company left (which is the one I use). Mesh computers has gone into administration as did Vadium, Commodore's new adventure into custom PC's and a lot of others.
There are people out there who still want custom PC's (hell I'm one of them), the issue is that some the profit margins aren't huge unless you buy so many parts at once from suppliers to save money. PC Specialist is probably the best company in the UK now. I have bought two from them (got one for my dad also recently). Their level of customer service is very good (which is why Mesh failed) and their prices are actually better than sourcing components yourself (which never used got be like that).
My new PC is a toy really. I bought it with the intention to upgrade it when I want to. It has some pretty high end stuff (which is already out of date). One thing I do like are SSD's... booting a PC up in about 10-20 seconds is quite good when you're in a rush to do something.
The one thing custom companies have over 99% of commercial companies like Dell, Hewlett Packard etc is knowledge. Almost all computers in PC World, Comet and Currys are terrible. They are slow, ineffective, poorly made and outdated. Unfortunately, these companies prey on those who don't know a great deal about computers and sell people junk. My dad went there a while back and asked for a computer that he could use his full HD camera & his DSLR camera on for editing. They sold him a computer without any HD graphics capability and a monitor which couldn't handle full HD even if the PC could. I didn't even let him unbox it and took it back. They still tried to say it was capable of full 1080p work... If more people were wise of their sales tactics and the state of what they are selling, I think custom PC companies would flourish more.