We have the Hi-Liner and I love it. We put DD into it when she turned 4 as she's tall and was at least 18kg. If you do go a booster that uses the cars lap-sash belt, make sure that it has an adjustable seatbelt guide plus an anti-submarine clip. Very few have the latter, which surprises me as I've seen many kids slumping down into their seats just in normal travel without the clip there. She's always sat very securely in it and by 4 could be trusted not to release the belt (same can't be said for her cousin at that age!). You know your child, if they're likely to muck about in a booster then invest in a bigger harnessed seat.