We've got one. Its pretty stable - stays on the chair when dd rocks about. And it fits all different sorts of chair, has two heights. We used it as well as a booster seat (high table, short child!). Dd has been using it on its own since she was about 2 I guess.
I suppose it depends on whether you think the novelty of a "grownup chair" would wear off or not. Ultimately he'll be on an adult chair that is a bit higher - so free to climb on and off, or twist round, or fall off sideways etc. And its more of a drop because its higher!