My dd is 17 months old today and is still very attached to her naptime and bedtime bottles. She drinks soy milk in them, but she always gets one first thing in the morning, before nap time, and before bed. Sometimes she wanders with it in the mornings, but after breakfast, that's it -- Mommy makes it disappear.
At naptime and bedtime, we cuddle and she falls asleep in my or dh's arms with it, but we've never allowed her to have it in bed. Like someone else said, I think of it like nursing. I wouldn't suddenly wean her at this age, so why take away her going-to-sleep source of comfort? She also won't drink soy milk out of anything but a bottle, and we're veggie, so I'm also hesitant to take away one of her major sources of fat and protein.
As for formula, I think that's your call. We switched when we ran out of formula around 14 months, I think, and really, I wouldn't have switched her if it weren't for the cost. Dd was on Alimentum, a hypoallergenic formula that cost $25/pound.
Mercifully, soy milk is more affordable.