I had to get rid of my king cab because of the child seat issue. Rear facing absolutely eliminated the front passenger. Forward facing was doable, but the baby and the person in front were uncomfortable.
Now, put the baby seat in the crew cab and the situation is much better. There are only two issues: one, the backrest of the second row is bolt upright. When the baby falls asleep, her head falls forward instead of resting against the headrest. You can place towels behind the lower portion of the car seat to recline it a bit more--which leads me to the second issue. The back of a frontier first row seat sticks out at the top, behind the shoulder area. It's right where her feet are. I've slid my seat up to give some space for now, but I know she's going to drive me nuts kicking the seat when her legs get a little longer.
All things considered though, these are small issues that can be easily dealt with. I'd buy the truck again in a heartbeat.