Very neat and very practical, but for smaller bikes, the legal issues get in the way!
I have a 50cc scooter, and am not allowed to pull a trailer as my bike is less than 125cc, BUT, stupid as it may seem, I CAN add a sidecar! Isn't the law an arse!
The reality of the situation is that a single wheel trailer is much safer for a small bike than a sidecar, as the wheel follows your line, whereas a sidecar runs a completely different line, catching in potholes, drains etc, makes the bike much more of an obstruction by forcing you to ride further out in the road and creates sideways drag which can be dangerous on light-weight bikes, but it is the only legal solution if you want to carry any luggage.
The U.K. law needs to be revised to allow small bikes to tow solo-wheel trailers, after all, you can have them on push-bikes!