yeah, I think most of us know there are "free" versions of S3d floating around - tbh, not really interested. 
the variable layer heights I knew about - that is covered in the manual, but the modifier thing I did not know about until yesterday.
printed this out yesterday, at my usual settings for knick knack type stuff - 3 perimeters, 10% infill....and certain areas did not turn out well due to not enough infill under them...

had I known about the modifier before printing, I could have increased the infill in just that area to solve the problem. As it was, I increased the number of perimeters to "fix" it, and it did, but drastically increased printing time by doing so. (not to mention the filament used)

I guess part of my problem was that I generally use Slic3r through RH, and as ysb said, the modifier thing is not available that way. But now that I know about it, I will be using it as a stand alone more often. 
SD4 #1 & #2 - Lawsy carriages, E3D v6, Rumba controller board, mirror bed plate, X motor fan, upgraded PSU & Mica bed heater
SD4 #3 - in the works ~ Folgertech FT-5, rev 1
Printit Industries Beta Tester - Horizon H1