Both of my daughters were exclusively breastfed and both went breast to sippy cup (no bottles) with no problems. My oldest daughter (now 3) started taking a sippy cup once a day at 5 months old (with just water....was trying to get her use to the idea). And when I started weaning her, it was no problem for her to take the sippy cup (with water...she HATED milk, she never would drink it and to this day, doesn't like milk). I didn't completely wean her from the breast until she was 15 1/2 months old, but she was down to only 3 feedings a day at 12 1/2 months old. With my second daughter (now 17 months old). She started taking a sippy cup at 7 months old (w/ water). But she didn't really start getting the hang of it until about 8 -8 1/2 months or so. Then when she turned 1, I introduced milk in the sippy cup, at first she didn't like it, but I kept trying (just like I did with my first daughter) and after about 3 weeks, she drank it no problems and now....she LOVES milk. I actually still nurse her once or twice a day. YES, I know she doesn't "need" it, and she probably could care less. It's more me....she's my last, so it's "sad" to be "saying good-bye" to that bond. Anyway, yes, I'm in the process of weaning her and she will be completely weaned in 2 weeks.
As for sippy cups...with my first daughter I used the Nuby (in the beginning, until she started to get more teeth), then moved her to the Playtex. With my second daughter the Nuby's didn't work...she figured out how to make a mess really quick. SO, I just started her with the Playtex sippy cups....LOVE that brand (I love the insulated ones...the only ones I'll use). They are the only sippy cups that I have found that I have NEVER had a problem with, and I've been using them for 3 years.
Good Luck!