My 6yo has been in a booster (Monterey) in my vehicle for over a year now.
In DH's truck, she just moved to a booster (PW) from a harness (GN) last week. I figured that a properly used booster was safer (than a potentially improperly used harness) as I found out that DH was letting her do the harness completely and NOT checking it. Like, ever.
Its funny how karma works, though. Due to the configuration in his truck, DD#1 needs help buckling the booster in his truck (in my vehicle, she does it by herself). For some reason, his female end gets wedged behind the PW when she tries to buckle it. So an adult has to help her anyway. Its kinda funny. If he'd bothered to check her harness (even by reaching around from the driver's seat would have made me happy), I wouldn't have put her in a booster in his vehicle. I asked and asked and he would.not.help her. So, I got her a booster and now he HAS to help her buckle the belt. She blows a complete gasket if the vehicle moves even an inch before she's buckled in (I swear, did NOT coach her in that!).
Long story short, yes, my 6yo is in boosters in both our vehicles now.
FTR, she MUCH prefers the Monterey. I know people here don't think it fits skinny kiddos, but she's super skinny and it fits her really well. I kinda wished I'd gotten her a 2nd Monterey instead of the PW, but I wanted to try out a PW anyway.
HTH