I know you'll probably think I'm bound to say this (as I work in mastering), but I truly feel that with the right mastering engineer, who is of the same mindset as you and understands your aesthetic, the process can be magic...but the key is proper communication, as with everything! I feel that if you find the right engineer to work with, it can just add that extra cohesiveness to your project (if you wanted it), a bit of extra sparkle and polish (if you wanted it), and the opportunity to hear from someone who is completely objective!
You're right that it shouldn't be overrated, there is a lot of hype surrounding mastering as a 'black art' when in reality it isn't! Due this hype, it creates a certain mystique about the process, which leads people to believe that mastering can rescue anything, no matter how poor the initial recording. Or in a more negative light, due to the level wars, it causes people to think mastering engineers are people who destroy your hard work by adding loads of bass and highs, slamming into the limiter for 6dB and calling it a day! I hate both of these things equally, but all I can do to remedy them is to try to produce good work, to always stay with the intent of the artist and the mix even if it goes against my own taste, and hope that people see me in a different light.
Of course it's possible to do great mastering on your own, there are so many great examples of this, if you know your speakers/room and equipment well, the mastering stage will only really involve the final QC listen, burning the master, and paperwork. Personally I feel the objectivity of an external mastering engineer can be a great aid with these tasks, especially QC, as when you work on something for a long time there is the tendency to sometimes be a bit lax, so the extra pair of ears can really come in handy in hearing glitches, clicks and that sort of thing.
And plus, who enjoys doing paperwork?
I do understand the additional cost of mastering can be prohibitive though.
Sorry if I rambled on a bit aha, I'm very passionate about mastering and I love to talk about it! Runningonair, if you'd like I'd be happy to recommend some good UK based mastering engineers to you, or if you were ever to think about using external mastering you could try posting on the gearslutz "Where to?" forum (http://www.gearslutz.com/board/where/
), you would get some great choices there too.
Take care everyone,