I got a new FNX40 for $450, not OTD about four - five years ago. $400 OTD is pretty good for an FNX you KNOW has 500 rds. max through it.
Remember it was built on the 40. The 9mm has the slide lightened up a bit. Still a great pistol. The G4 Glock's were built around the 40 so make your own conclusions about a 9mm built around a 40 (G4 9mm Glocks come to mind).
Mags are great. Very strong mag springs and the 9mm's hold 17+1 I believe. The 40's are 14+1. So you get a 4" tube that holds what a 4.5" Glock 9mm holds. Awsome grip angle IMO, better than Glock for me.
Mags are more expensive. I'd probably buy it if I shot 9mm considering they are $600 - $650 OTD new now. Glad I bought mine when I did. One of my favorites, it's always at my side at home and in the truck. There are better IWB pistols for me, though a SA/DA with safety works w/o holster on the nightstand very well.
Get it, sell it for what you paid if you don't like it.