The 18-200 is a good all round lens... but it just wouldn't produce the quality of the 10-20.
If you do go the 18-200 route i would either keep the 10-20 or get a nice prime lens, just so you have a quality lens in your bag.
I have the 18-200 + Sigma 30mm.... still not sure if the 18-200 is what I need as its a good all round lens that does not exceed in any area!
Take a look round my Flickr account for a range of shots.
Saying all that... its been a damn useful bit of kit in certain situations when changing lenses would be a pain! Such as when i was at the wildlife park... I was going from 35 - 200 mm all day long!
as for the 18-135... I would say that the 18-200 is a better lens, the VR above 100mm has been a life saver.
The lens does suffer from lens creep, but nothing major, plus it has full time manual over ride.