Personally, I couldn't wait until my kids were 40 lbs so that they could be in a booster seat. Now they can buckle themselves in (well, they could already do that) and unbuckle themselves. They LOVE that they can do that independently.
I'm one of those people that tends to assume if the AAP says it's safe for them to move to a booster at 40 lbs, then it was probably safe at 35 lbs but the AAP was just being cautious as many parents will jump the gun and move the kids a bit early. Kind of like when the speed limit is 50 mph, it's probably safe to drive 55 mph. The speed limit posted is usually about 5 mph lower that what is actually safe because most people will driver faster than the posted limit.
But I really did wait until they hit 40 pounds.
ETA - Momoftwo, there are carseats that weight 20 pounds? Wow! I don't think any of ours weighed more than 10 pounds. Still, at 45 pounds my child was in a booster seat.