We have one and my almost 2-yr old loves it! There are so many great brands out there, but we chose Wishbone http://www.wishbonedesign.com/#!/products/bike/details/
) because we wanted him to start using it early. It converts from 3 wheels to 2 as they get older and more confident. He started using it around 18 months in its tricycle form and we'll probably convert it to the bicycle form in the next 4-5 months once he's a bit taller.
Seriously, it's changed our afternoon/evening walks entirely. No more stroller or wagon; now all he wants is to ride his bike. He will very happily go for a 30 minute ride around the neighborhood... it's awesome!