Boosters are often called "belt positioning booster". They are often used because seat belts are difficult to fit properly in size ranges from children to full size adult. Children often times take the shoulder restraint and put it behind their shoulder. My husband used to be a law enforcement officer and has had much training, and witnessed many things first hand. He is also a race car driver, and performance driving instructor, and has a race car shop. Children who put the seat belts behind them for neck comfort are at great risk of suffering major injury if involved in an accident. Let's face it, they aren't called "on purposes", it's not like you can plan to be properly belted on that one day when it's going to happen.
Anyway, please please please, watch this video:
http://www.youtube.com/watch?v=azgBhZfcqaQ
One of the seats mentioned in the video is BRITAX REGENT. We just bought the seat and couldn't be happier. We travel to race tracks on the weekends alot due to my husband's business, and my daughter spends a lot of time in the seat. She says that it is very comfortable and she loves loves loves it.
I had been wanting the seat for her for a long time, but the timing $$ never seemed right. I just found it in a TARGET ad for $179 reduced from $279!!!! It was an online promotion only, and they had another promotion for free shipping on orders over $50! The seat installs very easily with the latch system, is super comfortable, and is good up to 80 lbs! Watch the other seats that say "up to 80 lbs" as often times they say in small print - "40 lbs as a 5 point car seat, and then converts to a booster to 80 lbs".
If the you tube link above doesn't work, go to you tube, search "5 point harness" and watch the first video that comes up posted by kcmillerfamily. I can't stress how important it is for people to know this!
Good luck!