Well, I have an old version of this - probably getting on for 40 years
old - and it's not great. I'm currently thinking about putting in a
wet system before it finally gives up the ghost. Apart from the
inefficiency of my old system, I would say the problems are incomplete
coverage (ie if it's not well designed, it isn't as easy to modify as
a wet system), poor control (opening and shutting vents isn't as
simple as setting a TRV and leaving it, plus the thermostat seems to
have a mind of its own, although that could just be my one), sound
transmission around the house through the ducting, inflexibility when
it comes to furniture positions (if radiators get in the way here,
remember that you need a clear path from the vent into the room) and
an overly dry dusty atmosphere. In terms of capital cost, should the
main unit fail, you can install an entire wet system for less than a
direct boiler replacement.
The positives are a fast warm up (but an equally fast cool down) and
the ability to circulate unheated air when the weather is very warm
(should that ever occur again this summer!). I am toying with the
idea of changing to a wet system, but keeping the vents and installing
a decent fan for a sort of pseudo air con system, although I haven't
found a suitable fan yet....
I hope that helps. In the end, I should think it's neither as bad or
as good as some people would tell you. One thing's certain though -
nobody wants to buy your house once they notice it, and that's what
will drive my change.