Initially, my husband and I were looking into getting a toddler bed, but as Lynn N. said, we decided that it was a better idea to use one of our existing twin beds with side rails. This was because we did not want to have to adjust our son to another bed after he'd outgrown the toddler bed, and because we already had a bed he could use. (Space was also an issue for us as we're in an Oak Ridge "A" house - if you're familiar with the term.)
Our son was just 2 when we transitioned him - and for us, it was a safety issue. He was climbing, and had already fallen out of the crib twice. (Trust me, NOT something fun to deal with at 3:00 AM!) His transition was seamless, and we were very lucky. We made a very big deal of his getting to sleep in a "big boy" bed and made a special shopping expedition to Wal-Mart for the side rails and special sheets just for him - and he got to pick what he wanted (within reason, of course).
The thing that surprised me was that once we had transitioned him to the big boy bed, and the bed was open enough for him to climb in and out of without posing a hazard to his health and safety, he didn't. He stayed put. He LOVES his bed and the transition has been extremely easy.
Personally - I wouldn't worry about getting a toddler bed unless you just want one or your child sees one he or she cannot live without. They're not terribly expensive, but why spend the $$ if you don't have to?
Good luck! Let us all know how it goes!
J. S.