I like my AT-X. That being said, I shot one of my FN SPRs yesterday. For $1,400 - 1,700 new (non-A3G) plus a $850 rebarrel (all in). It was and is still one of the best values to ever hit the market.
It's not as smooth as my AI, and the McMillan stocks aren't as adjustable. However, it isn't giving up anything in accuracy, reliability and pure shooting enjoyment.
When I eventually get another rifle, I'll certainly be starting with a base action and assembling it myself. Like you said, you can put an entire rifle together yourself with pretty premium parts for $3,500 - $4K...and have enough left over for a good optic to throw on top.