DMD,
I just finished the 18-135 WR review for Pentax Forums (sorry, it hasn't been officially published yet otherwise I'd link to it), and in it is a significant amount of attention spent comparing it to the 17-70 F2.8-4 "C," widely accepted as the best of the 17-70 variants from any brand.
Between the two, the bottom line is this:
Physically
+ 18-135 is weather sealed
+ 18-135 has significantly longer reach
+ 18-135 is smaller, lighter, and uses smaller filters
+ 18-135 is just as fast at AF (perhaps a hair faster?)
- 18-135 does not have distance markings for manual focusing
- 18-135 has a really unpleasant manual focus ring
+/- 18-135 only lags 17-70 by 1/3 stop for max aperture throughout similar zoom settings, 2/3 stop at most for a small amount (F5.6 on the 18-135 doesn't start until after 70mm)
IQ Wise
= Indistinguishable bokeh between the two at similar apertures
= Nearly indistinguishable center sharpness between both for shared zoom range (if there is a slight edge, it goes to the 17-70, but you have to be looking for it)
- 17-70 maintains excellent edge sharpness (especially after 2/3 stop closed down) while the 18-135 is very disappointing
= Just as easy to isolate your subject through shallow DOF
Doing the review made me realize one thing - unless you need edge to edge sharpness, I couldn't find a legitimate reason to skip the 18-135 WR in lieu of the 17-70. Of course, if the Sigma were weather sealed, that would be a game changer in this comparison, but it isn't. If you have a weather sealed camera, then imo it behooves you to get the 18-135 WR.
These shots were taken with the 18-135:
Hope this helps.
-Heie
EDIT:
I forgot to mention the pseudo macro abilities. The 17-70 offers .29 (1:3.4) vs the 18-135's .34 (1:4) max reproduction. Doesn't seem like a lot, but it is. The only major issue, though, is that at minimum focus distance, the 17-70 is literally within an inch (2.5 cm) of touching your subject. It was extremely difficult to use the 17-70 in that situation I found.
As soon as the full review goes live, I'll be sure to link to it.
Last edited by Heie; 08-01-2014 at 09:53 AM.