I've upgraded my kitlens to the 16-45 and I can say it's a very good upgrade. Sharpness/contrast are great with it. It has some downsides tho:
-Beware tho that the barrel extends towards wide and onboard flash does not work under 28mm approx. You might want to buy an offboard flash
-There is quite heavy CA if you pixelpeep however this can be easily removed by most programs.
The 16mm differs quite a bit from 18mm, however you won't notice on the lens itself.. It seems to differ not soo much but then again you just love the little bit extra wide if you do need it on occasion. Landscapes and architecture is where this lens shines.