One way to think of mastering might be to think of how much better you sound when you have the opportunity to play a live show with a sound person who is a complete master. Many sound people get a really good mix and good overall sound, but every once in a while you get someone who makes you sound like a million dollars. Mastering is doing that on purpose to your album.

I recommend it, as long as you stop the mastering engineer from doing anything you don't like the sound of. I also highly recommend enlisting the aid of a friend whose ears you trust and who has been through the whole experience to go to the mastering session with you.

Always work with an engineer who knows and loves acoustic music. A lot of mastering people who do more commercial music even out the overall levels by using a lot of compression. Compression is a useful tool when it is needed, but I would tend to apply it to specific instruments on specific tracks in the original mix if if that seemed the best way to clarify sound of the instrument. I've only used it on two or three tracks out of five albums. Acoustic music has a lot of dynamic range and too much (or any?) compression at the mastering stage, which affects all the instruments, can do serious harm in that regard. It often makes the album sound contrived to me, besides decreasing the joy of hearing the instruments get quiet and loud the way real instruments do.