The 35-100mm uses essentially the same premium construction as the 12-35mm F2.8. The rear segment of the barrel between the zoom ring and the mount is made of metal, as is the manual focus ring with its finely-milled ridged grip. The slim rubber seal that encircles the mount is the sole external clue to the lens's dust- and splash-proof design. Zoom and focus are both entirely internal, so the lens always stays the same length at every setting.
Both the zoom and manual focus rings are smooth in operation, but while the zoom is mechanically coupled, the focus ring is electronic. However such 'focus-by-wire' systems have improved immeasurably over recent years, and Micro Four Thirds bodies from both Panasonic and Olympus offer a manual focus 'feel' that's a pretty good facsimile of a traditional mechanically-coupled lens.
On the camera
The 35-100mm isn't a large lens by any means, but it still contrives to look bulky on most Micro Four Thirds bodies. It's certainly best-balanced on SLR-like bodies such as the DMC-G5, DMC-GH3 or Olympus OM-D E-M5, and like all lenses benefits from the extra stability of being shot using an eye-level viewfinder. It still handles reasonably acceptably on the 'rangefinder-style' DMC-GX1 or the Olympus PEN E-P3, but with these small bodies it's likely to benefit from the use of an add-on EVF.
The 35-100mm F2.8 is so small that it's barely bigger than its 12-35mm F2.8 little brother. Indeed when the latter is zoomed to 35mm, only a couple of millimetres separates them in length. Compared to a full-frame 70-200mm F2.8 it's absolutely tiny, and a fraction of the weight; but the trade-off is that the smaller lens can't provide anywhere near the same degree of subject isolation and background blur.