I haven't yet used them *from* LAX, but I really like Karmel. I've used the Express and SuperShuttle, and both have pros and cons, but so far Karmel really only has pros for me.
And they don't troll the airport for more customers (happened with SS at Ontario airport on a hot day with my 3 year old falling asleep at the wrong time), or sit and WAIT and wait and wait (like SS has done to me at SNA), so that's lovely. Nor do you have to stop at EVERY terminal at LAX like you experience with the Express bus (it's only Express during your time on the highway, LOL).