If you're looking for something to use for more traditional Illustrator purposes (graphics, identity design, illustration), you might want to look into iDraw, which is less UI-focused than Sketch, though both are quite powerful indie Illustrator replacements.
Pixelmator makes sense if you're actually working with photos, but just like Photoshop, I wouldn't recommend it for UI design.
I think Pixelmator is an excellent replacement for Photoshop, but right now Sketch is more of a replacement for Fireworks than Illustrator in my experience.
