There are 2 versions of paint jobs. It really depends on how you want it to look when it's finished.
#1 - Home / do it yourself, if you're like us, your standards go down significantly if it's a do it yourself project. i will allow some paint marks on the ceiling/trim and poor cut lines so it only costs me $100 to paint the whole upstairs, even if it takes for freaking ever and is a huge pain in the ***. You might say I'm cheap. If we were to show our house for a sale, I would have to go back and correct some areas, because it just looks bad if you look closely. If you are picky about what the outcome will look like, this is NOT for you. Again, if you can lower your standards, should be good to go.
#2 - You get what you pay for with professionals - it's done quick, and looks much better than do it yourself in my opinion. Most every professional job I've seen has great cut lines, no extra paint on the ceiling or trim, and looks like it got it's full 2 coats with good coverage. I think they use better than wal-mart paint and if you can afford it - well worth the money spent.