I have a Nikon 18-200 on my D5000 and it's my most used lens, replacing the 18-105 and 70-300. I also carry a 35 in the bag for when I need something fast (low light or shallow DoF). Whole kit fits in a Toploader bag so very convenient (although I'm moving over to m4/3 for even smaller kit...)
That said, the 18-200 is quite expensive for what it is and the zoom creep on earlier models may annoy some.
An alternative is the 18-105, which is cheap as chips and not at all bad. Far superior to the 18-55 IMO (more useful focal length range, doesn't rotate on focus so CPL friendly, instant manual focus override, etc). Not quite a superzoom, but 105 might be long enough for a 'walkaround'?