Cutting the corners so it connects right is harder than it sounds. Beyond that, it's not very hard. You don't even need a pneumatic nailer. A hammer, nails, and a nail set would do just fine (if take a little longer), as long as you're careful not to ding the moulding.

On the other hand, does installing it include painting it? A good painter will fill the nail holes, possibly twice, and paint the moulding, along with the rest of the wall and ceiling several times. Going back and doing that yourself could be a big burden, if they won't paint it, or you had to do it after the painters came. I'll guarantee that painting will be the biggest burden.