L.W.
M.,
I just put my 3 yr old in a booster because he is big for his age and was about to outgrow his convertible seat. I was told that it is fine to put them in a booster as long as they are mature enough not to squirm around in it. The seat I chose was rated 4 out of 5 for safety. Unfortunately I didn't have the option of being able to afford a Britax seat that holds them until 65 lbs. The seat I chose is under $40 and actually is rated a lot higher than some of the more expensive boosters. I chose the Evenflo Big Kid seat. My son is much more comfy in it and no longer has to fight for leg room. I drive a 4 door sedan so he didn't have any room to put his legs down in a convertible seat.
Here is a link with some ratings. I like this site because it rates boosters in safety, ease of installation and other categories!
http://www.nhtsa.gov/portal/nhtsa_eou/info.jsp?type=booster