matplotlib for R user?
This is a tough one to answer.  
I recently switched some of my graphing workload from R to matplotlib.  In my humble opinion, I find matplotlib's graphs to be prettier (better default colors, they look crisper and more modern).  I also think matplotlib renders PNGs a whole lot better.
The real motivation for me though, was that I wanted to work with my underlying data in Python (and numpy) and not R. I think this is the big question to ask, in which language do you want to load, parse and manipulate your data?
On the other hand, a bonus for R is that the plotting defaults just work (there's a function for everything).  I find myself frequently digging through the matplotlib docs (they are thick) looking for some obscure way to adjust a border or increase a line thickness.  R's plotting routines have some maturity behind them.
I regularly make figures (the exploratory data analysis type) in R. I also program in Python and was wondering if there are features or concepts in matplotlib that would be worth learning. For instance, I am quite happy with R - but its image() function will produce large files with pixelated output, whereas Matlab's equivalent figure (I also program regularly in Matlab) seems to be manageable in file size and also 'smoothed' - does matplotlib also provide such reductions...? But more generally, I wonder what other advantages matplotlib might confer. I don't mean this to be a trolling question. Thanks.