DD has one and likes it but it is a bit hard for her as we have no footpaths around her and practicing on the grass is near impossible. Also, not seeing other kids use them, left her a little confused in the beginning (I can hardly demonstrate )
I''ve seen kids in Germany use them. Most kids have them over there and they seem to have so much fun with them. We''ve just spent 6 weeks there and DD was using a neighbour''s bike and is slowly getting the hang of it. It definitely helped seein other kids use them.
I think they''re great and would recommend them. I have one that you can add pedals to at a later stage (like the Glide to ride bike from playskool, but not so wide where the pedals will be mounted).