Ideally, a wind turbine should be 20m (70') above any obstructions to keep it clear of any turbulence, but will work with less.
If you have a very exposed hilltop location with no obstructions you would likely get useable power with quite a short mast (although 9ft would be very short indeed). And the shorter the mast, the more likely it is that the mill only works with a stiff breeze from a particular direction, and spends the rest of the time doing nothing very much.
Windmills are one of those things that benefit from economies of scale, hence the tendency for commercial mills to get bigger - i think the largest currently available are 15MW, with blades over 700' in diammeter.
I think in wales, you can have a windmill under permitted development if it's overall height is less than 11.1m (there are other conditions too), so if i were having one, I'd try and be as high as i could within that to avoid the hassle of planning.
What i opted for instead is to buy shares in a commercial scale cooperative wind installation. (Google ripple energy), but then i don't really have a good site for a wind turbine anyway.
As to 'are they worth having'? well it depends on what your motivation is. If your looking to save on energy bills you might be better off with something like ripple energy cooperative. Or solar panels. If you're wanting energy independence from the grid, or you just fancy the challenge of building your own windmill then go for it. If you think your location is good for reliable, stiff breezes from several directions then it may well be worth it, but I'd still try and get it up as high as you can.