Why I Don't Master My Beats or Music with Reason
First, I must make it clear that while Propellerheads Reason 4 (and above) does have the "MClass Mastering Suite" of devices, I do not recommend that you master your own music within Reason. Mastering is a process that should be done separately from your production/mixing work. it should be physically separate - in a different location - and a separate individual should master your tracks for you - this gets fresh, independent ears on your tracks.
That said, i do use the Mastering Suite within Propellerheads Reason 4, but not in the "mastering" sense. I use these devices for fine tuning my mix - so, I'm still "mixing" with these devices in order to get MY final mix (before I send it to be mastered).
I'm glad that you think that my beats sound really good. Most of the time, you are hearing my final (un-mastered) mix that I did completely within Reason. So, my latest collections of beats and music demonstrate what can be done in Reason. high quality mixing can definitely be achieved within Reason, but it takes a lot of time and trial and error to develop your ear for improving your "mixing" skills.