We have a converter crib. It just converts to toddler bed from crib, doesn't go to full size bed, but I think those complete ones are really great!
I love the coverter/toddler bed. DS slept in that bed until he was 3 1/2 and would have been quite comfy and happy to stay there for a good while longer (maybe until he was even 5) but DD was coming so he is now in a twin and she is in the crib/toddler bed for the forseeable future . . . unless we were to have another or she outgrows it (age 5?). It made a great transition bed, especially for DS who was quite attached to his crib, never once tried to climb out and slept in it as a crib until he was almost 3. He liked it as a toddler bed. It has two wood sides that go on in place of the crib walls and with the mattress low, the sides go up a few inches higher than the mattress. He never fell out of it (though he's fallen out of his twin a couple of times!).
An added bonus to having a conertible crib is that if your bedding and decor isn't too babyish, you can then use it for years. If DD does stay in the toddler bed until she is 5, the sheets, quilt, etc will still look really cute and not like she is in a nursery still. Since you're using the same mattress you end up getting a lot of wear from your bedding, room decor, etc.
Hopefully though, DD won't be able to stay in her bed until she is 5 though . . . I would love to have another baby at some point. Fingers crossed, maybe someday!