I love my ProSport. DS (5) rides in it as his full-time seat in the main vehicle. I prefer it over the Regent that we had in my car, but moved to DH's. It loosens and tightens like butter. DS can loosen it, buckle, and just about get it tight enough himself. I always give it a final yank just to make sure, because he is pulling up, but he's usually got it pretty close.
It is LATCHable in booster more. If you search old threads here, you might get conflicting info on that because it's a (relatively) recent change. They used to just allow top tethering in booster mode.
Install is super easy IMO. Know that a shoulder belt could cross where the shoulder harness comes out and move it a little. Recaro says it's okay, but if you like, you can buckle but not tighten the seat belt, unhook the harness and let the seatbelt pass to the other side, then rehook and finish installing. I don't bother because I can always get the other side to pass the pinch test easily, so it's not pulling so much as to make one side tight and the other loose.
I have the lumbar wedge in mine. It's a separate piece of foam that Recaro will send for free. Again, the open belt path allows the seat belt or LATCH strap to lay in such a way that it's raised away from the shell a bit, and I could see it being uncomfy. The foam piece fills that space nicely.
I've tried DS in it as a booster. I think it would be very difficult for a kid to buckle, but the belt placement is amazing. I love the torso and head wings both for supposed SIP and for sleep support.