I'll try to keep my DD4 in her's for another year or so, I'm afraid that by five, if not younger (she's quite tall), she'll be too big for her Decathalon. We're running out of space in the car as it is, and the kids will only get bigger. We do already have a backless booster that we got a few months ago for rental cars, DH's truck, etc. She's not real fidgety, and though I do feel safer with her in a 5pt, it's just what works. Obviously, an accident can happen anytime, but since it's only used ocassionally I feel okay. I guess the real answer is to switch when your child is not only big/old enough, but when you personally feel alright with them sitting in a booster. If you don't feel comfortable, why rush it? 
