Just to share...when I offered dd a sippy (maybe 9 or 10 months old?) I was told by my sister to remove the little thingy that makes them have to suck iykwim. My sister told me that with that little thingee, she's have to suck really hard, and that it was exactly like a bottle that way. It made sense, and so that's what I did.
Also...I find it fascinating that thru this thread, the Moms with older kids (preschoolers and such) are all pretty much saying that it's no big deal to keep the bottle for a while longer.
Makes me smile...the way that once your "baby" is no longer a "baby", none of that stuff seems to matter...and all of the stress we felt about it has vanished and we're like "Let him keep the bottle!!!" Funny.