The Laptop isn't like other shield boosters for a few reasons: it is made of EPS foam, not hard plastic covered in soft material (therefore absorbs crash forces better); it fits very snugly against the child's body, and fits high on the torso (shield boosters allow a gap between the shield and the child, space for a child to fly forward in a crash--and they fit fairly low on the child's torso, room for a child to fly forward/down in a sort of jack-knife).
The Laptop is great for lapbelt-only spots but can only be used for children 30-65 pounds. The tips of the child's ears must not be above the vehicle seatback or headrest.
While it isn't particularly wide, it is difficult to use in some three-across situations because you must buckle it each and every time (like a booster), and the seatbelt must go OVER the Laptop, through the red belt path, and then get buckled. Some children have issues with the snug fit, and even my dd, who likes the Laptop overall, complains when she has to ride in it for any length of time (say, more than 20 minutes).
Still, I love it for what we use it for--the middle of dh's single-cab pickup and in my MPV's lapbelt-only spot when I have a whole van-ful of kids/people. It is compact and fairly lightweight and works in lapbelt-only spots, so it has been a lifesaver for us.
They are hard to find though, and even harder to find new ones or ones with a history you can trust.
Pics of my dd in the Laptop at 43 inches/43 pounds/3 years and 10 months old: