@eishiya give him a little credit. I don't see anyone complaining about those round cactus people from Super Mario Brothers 3. 
Looks like a cartoon to me... 
Still, eishiya has a point about the shading. I don't have a problem with the form of the cactus itself, but the pot is a little misshapen. Great job, wtxy. Keep it up and pretty soon you'll be pixeling those prickly pears in your sleep. 
I think you've given it a bit too much volume with the shading; it looks like a bunch of round bits, while the sections of an opuncia are flat, especially where they join up with other sections.
Thank you for your input, really helps out a lot :D only just started out here.