In my experience with the 18-135 the image quality is as good as the 18-55. The design is newer than the 18-55 so it -should- be better in the image stabilization part as well as manufacturing refinement part (not really a big deal). There is more optical compromise on the wide and long ends as it is still a consumer lens. Focus speed is as good or better than the 18-55. The focal length is quite good in my opinion, in others maybe not. I have two gripes, the first is zoom creep (creepier than a 40 year old guy with a mullet at a high school dance or something like that), and the other is minimum focus distance which is very poor.
Overall I would give it the same rating as the 18-55.
Mine is for sale if you want it.
The above opinions are for the old version not the STM version.
XS / 18-55 / 18-135 (for sale) / 70-200 f4 L / NissinDi622 / Manfrotto MY 7302 Remember, ALWAYS think for yourself.