Take a look at the eham reviews of the Tak-tenna and you will see that hams love it. It and the Isotron are the same thing - a short feedline current excitation device.
It's an interesting phenomenon. If you sold a simple end loaded antenna with a spiral coil out on one end and a feedpoint at the near end, everyone would obsess about a need for radials and never buy it.
You build the same antenna but move the feedpoint up to the hat end, and everyone buys it and reviews it well because it's being sold as a self-contained antenna even though it needs radials down at the bottom of the coax exactly as much as the former version.
I love how a lot of the people who are happy with 40m versions of this type of antenna have it mounted on top of a 20-30 foot mast :-) Probably makes it as good as any other 40m slightly loaded vertical that's mounted over a bad ground system... which, for someone who thinks they can't have an antenna, is quite good.
But if they went a little further and laid radials in the yard and just used the 20 foot stick as a 40m vertical, it would work GREAT.
Ah well, fun to play with it and learn what it does. Just don't forget the current meter on the feedline as part of the experimenting ;-)