@Noodle: Hello there. I'm really glad you liked my picture and I thank you for your critic. :)
I could not really tell the differences between SAI and OC since I never used SAI in my entire life. But I can tell you a few dfferences between OC and PS I spotted:
(NOTE: I'm using the free version of OC, the 1.1 one. Some things might have changed since there are lots of other versions for sell which must be containing lots of other options.)
. PS is a much more complex software than OpenCanvas: editable brushes in PS are simple round ones in OC (the only thing editable is the size), layers have lots of options in PS while the ones in OC are the dumbest ones I've ever seen (that must be one of the major problem of the program), filters, etc. In OC, you keep it simple: brush, pencil, color picker, selection, line, zoom and hat's pretty much it. It's very easy to use.
. OC has got a very "traditional" feel to it: the colors are easilly blendable, just like real paint, which is not the case at all in PS.
. One really interesting option of OC is the function "online drawing", in which you can draw with someone else on the same canvas and at the same time. Note that this function is only available on the 1.1 version of OC.
. Finally, OC is free. Well, the old version of it is... But still: it's free. (free version is the 1.1).
Conclusion: OC is a great program for sketching, simple and confortable, but I'd say it would be better to use PS or an other software to make big precise projects. But you should really try it out!
Here are some more links if you decide to try it out:
http://browse.deviantart.com/art/OpenCanvas-1-1-Tutorial-BIG-63578393 - Basic options
http://www.deviantart.com/download/22645124/oc1networkingtutorial.pdf?token=2a8d09348a361962e161fdf467b4e60ed3562963&ts=1367344810 - Guide for network painting.
That's it. Sorry if you find any mistake. Keep me informed if you like the program and do stuff with it. :D
See you around!
@ChamelNeon: Thank you too for your nice comment and yeah, I should have fixed the legs. :P