I've used STP a couple of times and found it rather buggy and limited - although the limitations were due more to the fact that I have a lot of third party plug-ins for my personal Pro Tools system.
And that is a large part of the equation, the tools available to you. Yes, STP will do the job. The EQs are okay as are the reverbs, although the 'verbs tend to be CPU hogs (not that some of the plug-ins I use are any more frugal). What is equally important is your listening environment and your speakers. You need to be in a fairly quiet space, and it needs to have at least some sound treatment; otherwise you will not hear the audio accurately. And the speakers need to be relatively flat and accurate as well. And the system needs to be CLEAN; in other words you don't want to be correcting for noise created by the audio monitoring system.
Isolating the dialog from the background/ambient noise takes lots of time and patience. The whole job is to eliminate the noise without compromising the tonal quality of the dialog. Having copious, well recorded room tones (indoors) and ambient tones (outdoors) makes the job a lot easier.
First, make sure that you work non-destructively, in other words you can always undo what you have done.
Start by making an original production sound session and then never use it again except as a reference.
Do a "Save As" of the original session. I generally do 8 to 15 minute sessions.
In each scene you will checkerboard the dialog - each character gets his/her own track.
Do the noise reduction.
Eliminate all of the ambient sound between lines of dialog.
Pull in your room tone.
Mix to taste.
It's that simple and a lot more complicated. Yes, it's hard and sometime boring, but you'll get better with experience.