We didn't bother with a toddler bed - DS went right from a crib to a full bed. We transitioned him when he was a little over 2.5. He had no trouble transitioning, but we did leave his crib up in his room for a few days. He had no troble sleeping in the big bed at night from day one, but it was two or three days before he would nap in there.
We moved DS to a twin bed with side rails right after his 2nd birthday. We need the crib for the new baby and we didn't want him to feel like he was getting evicted by the baby. If it weren't for the new baby, he would probably still be in a crib.
We went from the crib to a twin bed when DS was 23 months. For us, he was ready to switch. We have bed rails on both sides of the bed, so its almost like a crib anyways. We told DS he wasn't allowed out until one of us came to get him and so far, he's listened.
Re: ? for Mom's with Toddlers
Brayden (5 1/2) born 12.28.06
Hailey (4 1/2) born 2.25.08
Taylor (8 Months) born 12.26.11