I can give you my reviews, but your experience may be different depending on a few factors. We have a Burley Kazoo (the single-speed version of the Piccolo) and love it. It's extremely stable and the accompanying rack allows for use of panniers while using the trail-a-bike. We tried a seatpost-mounted Alleycat first and there was just way too much play in the connection (also, we didn't have enough clearance for a rack under it). It was far too easy for my daughter to tilt the bike from side to side and throw me off balance. We have friends who had the Adams trail-a-bike and had a similar experience.
I think seatpost-mounted systems can work okay if you are a strong cyclist and there is a large weight differential between you and the trail-a-bike rider, but if you have the money and you want a system that works really well, I'd go for a Burley. The initial price is higher, but the re-sale value is much better (based on what I've seen on craigslist, used Burleys sell for at least 50% of the original retail price). As between the Kazoo and Piccolo, I don't know that the gearing is necessary; we just spent the weekend in San Francisco with our bikes and my husband had no trouble making it up high hills in his lowest gear even when it got harder for my daughter to pedal.