I've been watching them for years. So far, all the previous posts seem to mirror my thoughts.
I talked to a fellow in Kansas that has a teupen. He really liked it, and seemed to think it was just as good as any bucket truck, but a lot more versatile. Incidently, he had some bucket trucks, too, so he should be able to tell.
They have several distinct advantages:
1. They go much higher and many more places than your average bucket truck, although they are rather lame for side-reach, compared to an over center bucket. Think dead trees in back yards!
2. They don't cost for vehicle liability insurance, and can be trailered around on a light flatbed behind a pickup. A 75' reach bucket truck will almost certainly be a CDL required truck to operate, with at least a 2k annual insurance bill, not counting damage coverage or business liability.
3. The linear extension of both the upper and lower booms gives much more maneuverability than any bucket. You can thread through the branches better, and you can get where you want to be easier, providing you are within the operating envelope of the machine. They will even outperform a smaller bucket in tight quarters, too, since none of their booms are even as long as a 50' bucket.
Disadvantages:
1. Holy cow! Those cost way more per pound than any other large machine.
2. They are rather lame for side-reach, compared to an over center bucket.
3. They are not quite a fast as a bucket, and they certainly can't take as much abuse as a bucket.
4. The probability of hitting your own platform base is higher (and probably more expensive) than running a bucket truck.
The only way it seems practical to own one is if you are doing a large amount of big trees in small/crowded yards, you simply have a big operation and can afford a really neat toy, or if you just plain want one bad enough to justify the expense. A small but highly profitable tree removal business would do well with one, too.