We went through a firm called Sustainable Energy from just outside Preston and have a 5kw machine, an Iskra.
We have not had it that long so can't say whether the output will be what was forecast yet, but it looks like it will pay for itself within 10 years.
We chose it over a 6kw Proven as it will generate at a lower wind speed, and we hope generate more in the long run.

Things to take in to account:-
Planning permission for us was for 20 years, after that we have to apply again.
Needs to be 30m or so from any dwelling.
Feed in tarrifs may change, so the costs may change.
In high winds you may not get the full output as it will throttle back to protect itself (tested up to 130mph).

We have had a lightning strike which fried the rectifier and shut it down for a few days. We also had a failure of a slip ring which the firm are most upset about as it is supposed to be military quality!

The noise is about the same level as having a tree in the same location, except that the tower has a resonant frequency that makes a noise when it's generating about 900w.

See if you can go to an open day at an owner's site, that's what we did.