It really depends on the purpose of the finished product. For demos and song submission it probably is a good idea to get a handle on it yourself. Izotope ozone is a good package. Some of the Waves stuff is very good.
Perhaps for an album it is better to get the mastering done by a professional. Outside of the obvious advantages of superior gear and a professional set of ears, objectivity will be a big advantage as well. Chances are, you will be too close to the project and an objective point of view will be a huge advantage.
Whether you get a pro or do it yourself, it is a good idea to assemble a couple of commercial CDs with a similar style and sound to what you are trying to achieve. If you do it yourself, it will give you something to compare your progress to and if you go to a pro, it will make it very easy to explain to him.
If he can hear what you want it will make his job easier and it should mean that you get a better result.