For what it is worth, I am glad we have them. We have 4 kids so I get a range of ages on it depending on what seating we have available.
If I only had 1 (maybe 2) I'd probably not bother. The caveat would be if they seemed like they would be slow to learn to ride or less athletic riders. Meaning, you'd have an older child on the TAB with some frequency rather than on their own bike.
Our goal was to get our son off the t-a-b this summer (when he turns 8) and hand it over to his sister, but his camp location is 4 miles from home, so it's 45-50 min riding there at his speed on his bike, vs 15-18 min riding there on the t-a-b at my speed. Depending on my schedule, I often don't have that extra half hour at one end of the day, the other, or even both, so the t-a-b is definitely getting a workout right now.
I do wish we had a singlespeed rather than geared one. It was just the one they had in stock when we 1st got the need. The gears are a distraction and added source of vibration and stuff. If it was one speed he'd probably contribute more to pedaling his own weight, which I could really benefit from especially like today at the end of the week...
I find the single speed is good enough on typical climbs - the most important time to get pedaling assistance. But I don't recommend riding places where the adult isn't strong enough to pull the child up the hill unassisted.
On level ground or downhill, I find that I limit our speed for safety; I don't need a pedaling boost. He often just coasts.
When he gets older he might get bored coasting or spinning when we are traveling at higher speed, but I hope to have him on his own bike or upgrade to a tandem by then. At 6 he is just thrilled to be sitting up high like a big kid.
I have the Giant Halfwheeler seven-speed for my five-year old daughter.
I hear her using the gears behind me and can sometimes feel her kicking in a little more pedal power at times, but I don't think it's anything you should dwell on. I wanted the aluminum frame of the Halfwheel and the seven-speed was only a few bucks more than the single-speed, so I went for it.
From what I can see the Kazoo comes with a derailleur mount at the rear wheel which makes things easy if you want to go high end but to add gears, you need: a shifter, a cable and housing (long enough to go from the shifter, along the down tube, along the chainstay to the derailleur - just use a few "zip-ties" to hold it in place), a rear derailleur, a rear wheel with gears and a longer chain.
The least expensive solution is to find a 20" bicycle with gears at a garage sale. You get the shifter, the derailleur, rear wheel and maybe the chain for a fraction of what you would pay for the parts. If you want to go high end, you can put quality derailleurs on the Kazoo and the shifting will be much better as a result but the cost will be higher (again buying at garage sales will get the parts much cheaper but you need to know what you are looking for).
If you're mechanically inclined you can assemble it yourself or if not, drop by the LBS and have them assemble the parts. You probably have to drop by the LBS for the looonnng housing (even my spares bin didn't have that part) and a new cable.
Now whether you need gears or not is a separate issue:
My daughter and I ride out of a community named Hidden Valley, so every ride starts with a hill climb to warm up. So having her pedal in 46-26 means she provides almost double the assist that she could with the 16T rear sprocket that came with the bike...and I appreciate the assist. And the 46-12 means she can still be assisting when we are truckin' right along.
Now, it does take some (lots) of vigilance from me to ensure she is in the correct gear.
p.s. "thumb" shifters seems to work better than twist shifters...something I learned the hard ($$$) way...
Don, thanks for the feedback! I should have updated -- we ended up w/ a Piccolo (got lucky I found an almost new used one!). I'm glad for the gears -it's been a learning curve for my son, but I can hear/feel when he needs to shift so I've been prompting him to shift (it is twist shift). I think that it will make the transition to a geared bike in a couple of years very smooth
I picked up a Piccolo off Craigslist this summer when my girl was five. It didn't take long for her to figure out how to use the twist shifter, and at times can propel the two of us at 8-10 mph.
At first she thought that the smaller gears were too hard, but I told her that peddling harder made her faster.
Worked for me.