Hi Smurf
We've got one, used it for my daughter (from 10 months) and will do now for my son.
It does require a more legs out pedalling style but I have always found the better balance than a rear seat compensates for this. It is great to share the ride with the litte one, you can talk and see the same things.
The position of the seat means that the childs weight is in the same position as yours, meaning the bike is well balanced, this means you can be a bit more adventurous in your your choice of trail.
Fits to any bike, our goes on hardtail and Full suspension.
Downside is that it fits on a mounting bar that runs parallel to the top tube, the bar is pretty heavy. This means that if you use the bike for other rides you'll want to take it off, this means re-mounting it each time you go out with the family which is a 10 minute faff.
Helmet from Halfords
Ben