i agree with nick regarding documentation.
i have been using mgraphics exclusively since max6 release and it is mainly fantastic. plenty experimentation required though.
the thing about canvas in max6 is that it is really there as a convenience right now, as in max6 it is just a compatibility layer between canvas syntax and mgraphics. you can see this by looking at the file
/Max6/Cycling '74/jsextensions/CanvasExtension.js
i expect ('my guess is') that it is there now as eventually canvas will be more fully integrated into max. anyway for now, canvas will be significantly slower than a native implementation, and slower than mgraphics.
ej says (i seem to remember) that his ej.function object got a 250 % speed increase porting from sketch to mgraphics - that is significant. there are still a few things in sketch though that are useful. i am no expert though, just a tinkerer.
my personal huge wish / feature request is that one day cycling74 do away with compatability layers and get rid of jweb and fully port/integrate the entire webkit into max. i think this would be very powerful and very useful.
2cents.