My 15 month old loves his NUBY silicone top cups! The only advice I can give is don't put them to bed with a cup. I have seen from a family friend the how quickly their teeth can rot and possibly have to be pulled which can make learning how to talk hard.
I do like the other advice though about teaching them with an open cup so as not to have to wean again. I take a little time each day on the kitchen floor with a towel and let him try drinking a little from a cup, then I put another couple sips of water in the cup.