I have used one since my son was just over 2 years old, he is comimng up to 4 now and we still use it – even though his feet are getting too big for the plastic footwells.
They seats are quite small and the straps only just fit him when we first had it. Now he doesn't have either the straps or the centre pillar arrangement – that got taken off about a year ago. It's very good for small kids but now he is older he holds on to the handlebars.
I have always reckoned that he is safer than in a back seat, I had one of those for a while but it unbalanced the bike – also couldn't see what he was doing! With the centre seat we chat away and it's a really joyful experience. I have gone out at least once a week for all the time we have had it – average ride is about 8 miles and he is comfortable. It gets left mounted on one bike – I would hate to have to fit it every time – and although you can leave the mounting bar on the bike it is a bit heavy and reduces the stand-over quite a bit.
The other problem now he is larger is that his helmet is resting about 2cm under my chin!
I am considering getting one of these as a replacement – but they are a bit pricey!