I bought one today. I'm pretty pleased. I bought it for my 4.5 yr old, who is almost 38 lbs and 43 inches tall. We've been working on him learning to ride in a booster properly, b/c his grandmothers can't buckle/unbuckle the combo seats like the Maestro/Secure Kid/Nautilus. It's for occasional use, and he understands how he must sit and does very well.
This booster folds really easily, once you know how. It has really good belt fit. It's SUPER lightweight. And it is rated from 30-110 lbs, which is unlike some of the others out there that start at 40 lbs. That's one reason I didn't pick up an Evenflo Amp highback--it starts at 40 lbs. That and the belt fit didn't look as good on the lap belt.
The only part I'm not thrilled about, but it is a plus in some ways, is that it can be tough to get the shoulder belt in/out of the guide. But my kid can't remove it from the guide while he's riding, once I've put it in, so that's good for new booster riders, little stinkers, and grandparents!
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And yes, I agree a 4 yr old should stay harnessed whenever possible, but for a travel seat and if it's not, this is a good, well fitting option.