I use Jomtien, I have the money in the bank and, being retired, I have the time. Having said that, I use the agent in the compound to complete my forms, do any photocopies needed, etc. Then take them in myself. Cost is usually a couple of hundred and I have never had issues with the forms in immigration - I think that, seeing their paperwork, the officer pretty much rubberstamps it, he knows it's correct. Worth the (small) money to me.
I know many who use agents because they can't, or don't want to, put the money in the bank. I've always thought that it's dodgy though, a "Big Joke" type in charge could cause some serious grief there.