Tell me everything... I'm interested in the one in swap and couldn't find much when I searched. Is it really awful for an everyday seat? I am in need of a 3 across soon and I don't love my radian. The seat would hopefully go in the middle (so no LATCH) for B to use. She is about 53ish lbs and is almost even with the top slots on the XTSL. Will she have any room? How does it puzzle with other seats? Would you use it as an everyday seat? How hard is it really to install with a seatbelt?
Seatbelt installs are rumored to be rather difficult. Installing with a seatbelt also supposedly causes you to lose some harness height. (I say "rumored" and "supposedly," because although I have a Go--the SafeGuard model--I've never attempted installing it with a seatbelt. I do believe the people who claim it, though.)
I honestly don't know how much, if any, harness height a kid would have if she's even with the top slots on the Radian. Maybe a little, but again, you lose some height with a seatbelt. Supposedly.
I think it would be fine for an everyday seat, although I'd probably prefer something with a hard shell. Not something I'd lose sleep over, though.
In my limited experience with it, it's not as narrow as one would expect. The contours made it not puzzle well, either. I forget what combination of seats I was doing, but the Go didn't work next to whatever I had (probably a Nautilus) so I had to switch things around. That was only trying it next to one seat in one car, though, so I'm sure other people's experiences may vary.
A RideSafer Travel Vest might be another option for you to look into.