Last fall I went for the Tamron 18-250 and it hasn't disappointed. Last week, however, a Ritz-closeout deal could not be refused: a Quantaray/Sigma 18-200 for an excellent price. After several tests (both side by side and separately), I find that the lighter weight, slightly faster focus and equivalent image quality has me leaning to the newer lens. I never thought I'd say the 18-250 was expendable.. but there it is.
The primary good news from this is that I can swap out the 18-250 and finance my 'final' lens - one of the 17-xx models most likely, or perhaps a 16-105. The 17-70 has some advantages, since it would focus close & match up with my Minolta 70-210 - plus it zooms 'backwards' like the 18-200!

A nice iris shot from the weekend with the 18-200, no PP:

Original (for zooming in) + others at http://picasaweb.google.com/alphaPDX/qsigma