2008 Prius
Parkway (old style), RF Radian XTSL (with the new boot), Parkway (old style)
I initially installed the Radian using the locking mechanism of the seatbelt. However, by the time I got the belt tight enough to install the seat securely, it was also tight enough that the shoulder portion was tilting the Radian (also, the little plastic trim piece where the shoulder belt comes out of the backrest was being pulled out by the force on the shoulder belt). I ended up reinstalling the next day with locking clip, and it looks much better. Buckle stalk has two full twists; locking clip is sitting flat inside the belt path. This was not an easy install, but ended up solid. Wish the seat would go more upright, as it lightly touches the back/inner edges of the front vehicle seats when DD is in the carseat, and Prius manual doesn't want anything touching. We may try moving vehicle seats forward a notch.
The boosters are a pain to buckle, but doable. I'm sure I'll get quicker with a little practice.