I've pretty-much always "built", but not too long ago I decided to "buy". I found a couple of Lenovo i3 deals at Staples, that were a bit less than I would have paid to build them.
I figured I could upgrade the PSU, and add a video card, and have a gaming rig. So I did, I put in a PSU, upgraded the RAM from 6GB to 8GB, and even added an SSD, and thanks to the MS refresh media, put on a nice fresh copy of Win8.1.
I used it for a little while, it was speedy, being an i3 and all, but I had some issues with the GPU, a 7790, and Windows 8. They didn't get along.
Ironically, perhaps, I sold the PC to a friend, and then later on, sold him the exact same video card. Only that's after I upgraded the OS from Win8 to 8.1.
Well, that might have been the reason I had trouble with it (and/or the updated Catalyst Omega drivers fixed the bugs that I had been having), since AMD's drivers support Win7 and Win 8.1, but no longer support Win8 itself. (Nor Vista.)
Anyways, my friend is happy with it, and hasn't reported any problems with it. He's been gaming with it too.
If I had it to do over again, I wouldn't "buy". The low price is seductive, but if you have issues with it, like I did, it's not like it's easy to just swap out the motherboard. And if you did, you would have to re-purchase the OS. Which throws the savings out the window.