I have two of the non-tethered boosters. The Infa-Secure ones. We don't use as main seats but use when we need additions, or we have mum and dads car which has no tether in the back seat. We took to Darwin x 2 and used on holiday - I prefer this to getting whatever the car company has left - strictly speaking DS is not old enough, but he is heavy and tall enough (heavier and taller than plenty of 4 year olds) and sits still enough that they are a safe option for him.

I like them - they are not that different to the tethered booster really - I like that the arms on the Infasecure one keep the belt low across the top of the legs. The kids seem to find them comfortable. They fold flat so go in the boot easy (or in luggage I suppose although they are not small). I didn't actually look at the other options non-tether boosters though as I quite like InfaSecure as a brand so I couldn't offer any useful info in terms of comparison.