The barebones Image source: Tony Mac x86

I've been wanting to build my own machine for quite some time. Ever since I found out that the only way I could upgrade the GPU on my late-2009 iMac, was to use the GPU from a 2011 iMac, for around £600. This was in 2013. It's taken me a year of deliberating and saving to get to this point.

Pros Cons
For the same performance, it can be cheaper to buy your own components and assemble yourself Apple provide everything in a neat package and it works out of the box
Future hardware expandability or upgrades are easier You are the support; Apple won't help you with Hackintosh issues
You get what you want as you select all the components yourself Many suppliers, troubleshooting issues can be time consuming
Experience of assembling your own computer - awesome! Future updates to the OS could kill your system

Sources: Me, MacRumors, AndrewMRiv, SLR Lounge, Tony Mac x86, Next of Windows.

There are many more Pros/Cons, I'm sure, but I ordered all my components and the build has started. Posts to follow.

My parts list can be found at Part Picker