I recently bought a trail-a-bike copy for my 5-year old son.
Summer vacation is just a few weeks away and I wanted to take him with me on some trips, so I thought I'd give it a try.
Adam's Trail-a-bikes in North America are fairly expensive; $245-320CDN, but this one cost $4,800NT ($165CDN). Mountain Equipment Co-op in Vancouver sells their own version for $190 + tax, but it didn't get very good reviews:
I was surprised by how much the slack on the joint affects handling for the adult. As the kid pedals or moves, their body weight will shift position significantly and impact the balance on the front bike. I found myself "fighting" it all the time to where my hands and upper body were noticeably tired after riding. Having done a fair bit of cross country mountain biking, I probably have above average handling skills compared to a more novice recreational-riding parent, so I would encourage trying it out before buying. For occasional riding with the family, the trailabike still fits the bill and the MEC one is good quality and priced right.
I definitely noticed that my handling was affected by weight shifts on this trailer and had to fight to hold my line. Found it best to try and keep my upper body as relaxed as possible and just let my line wander a little. Was really great to be out riding together with my son though. We took the long way home from his school and followed the bike path along the Love River. Got quite a few surprised looks from people we passed and lots of smiles.
Gonna have to go helmet shopping with him this weekend, and get a proper bike helmet that doesn't slip back on his head.