My daughter has just turned 16 months. My ped wasn't too concerned with the transition, but I remember with my first daughter that we transitioned her around this age and it will be easier now than later. I am tired of cleaning all the parts of the Dr. Brown's bottles :)
My daughter does drink water from a sippy, but it is only ones with straws. She likes a fast flow (even from breastfeeding days) and so this is the only way for her to get a big gulp. I use the toss and go cups (I think by Playtex) similar to what one other said. I started introing during one of the meals in the highchair, because she likes to tip it over and such. Did this for a week and then finally she started to drink from it (still was on bottle for all milk for the day, this was in addition).
I finally looked at my schedule (because I am a stay at home mom) and wanted to change during a week when we had the least amount of things going on, so I could have consistency in the rest of the schedule. Then cut it off cold turkey, except for the bottle before bed. Gave her the sippy at the same times and locations as the bottles used to be given. The first five days she drastically reduced the amount she was drinking, taking in only 8 oz or so from all of the sippies for the day (was drinking 24 oz), so gave 6-8 oz before bed (used to give only 4 oz to top off for the night). Made sure she was well fed with other food, especially calcium and proteins to make up for less milk. Then all of a sudden one day she drank a full 6oz from the sippy in one fell swoop. . .that was our turning point.
She has probably reduced her milk intake overall, but is not cranky and is eating regular food fine.
So, I use the take and toss, when I know that I will be around to assist, because it does leak and so if it topples over than I can turn it upright. If I am on the way to pick up older daughter or such and need a spill prood, then I use a Nuby with straw sippy.
Good luck. Just be consistent. When you start stick with it. You can do it :)
E.