Hi C., we just made this transition this week with our 21 month old. I couldn't decide whether it was too early or not, or whether to get a toddler bed or not. He can climb out of the crib and has tried, he's at chest level with the mattress as low as possible. But most of all, he takes FOREVER to wind himself down at night (an hour), he wakes very early (5am), and he kicks and hits the sides of the crib which makes it sound like he is breaking the house down. And wakes our 7.5 month old an hour before she should be getting up. After lots of back reading on this site and looking at our own situation, we decided to get him the little tikes car toddler bed, off Craigslist for dirt cheap. We are fully aware of many moms' other opinions that toddler beds are a waste of money. But moving him to a twin right now is not an option for us (with our current living situation and travel schedule).
We also have had him sleep in a PeaPod Plus tent, on top of a queen bed. He did great but it was hot in there, and buying a peapod would cost you money too (we plan to use it for traveling and the plus one is good for kids up to 4 years old or older).
Last night our son slept on the car bed for the first time, after 3 nights on a crib mattress on the floor. He stayed in bed the whole night! He was asleep in 5 minutes after I left the room! And when he woke, he stayed in bed and waited for us to come get him (as always - he does this on the cot at daycare too). He was pretty wound up about the car - "my car" "my bed", so we spent 45 minutes doing the wind down routine instead of our usual 15-20, and I let him lead when he wanted to get into bed. I also had the crib up and with a mattress in it, so if he chose to sleep in it, he could. We will leave it set up for at least a few weeks so he can choose it if he wants. I hope this helps....