My baby is nearly 10 months and has always refused to take a bottle, and has also refused to drink from a sippy, so I decided to try a straw. So far it's been a huge hit. Within a few minutes of handing her the cup, she had already figured out that she could get the liquid by sucking on the straw!
It has leaked once when the cup was smashed around but I hadn't thread it properly - now I just give it a quick shake to make sure it's closed tight before handing it to the baby. I was also shocked at how much of the liquid she was able to drink using the weighted straw - she held the cup herself and when she handed it back to me it only had a small drop of liquid left in it.
My only negative actually applies to all straw sippy cups - the inside of the straw is difficult to clean well. Filling the cup with hot water, closing and shaking it causes enough pressure to shoot water out the straw, so that should be ok if you wash it right after use.