It took me ages to decide on this lens as I need something compact and still with high image quality and clarity. Though the lens is not as compact as you would expect from other micro four thirds, it is still smaller and lighter then Canon and nikon 70-300 mm IS and VR variants, and atleast as sharp (I have tried excellent 70-300 VR on Nikon D90) and Best IQ level Sigma 150mm macro on Oly Pen E-P1 with the adapter, which was very very heavy.
+ IQ Excellent on all focal lengths (I mostly used 100- 210mm, very few at 300mm) and most apperture I tried (never closed it under f 1:8). Even works wonder at f 1:4.
+ Mega OIS is excellent, but I suggest turn it off if you are taking any shot faster than roughly 1/125 sec or on tripod, it will save you some battery life and probably better IQ in few cases, just like any other IS system.
+ Excellent build from plastic and very good rubberised focus and zoom rings, that you can always expect from Pany. Same or better than 14-140mm kit lens.
+ Very accurate and fast AF on GH1, for this focal range.
- Price is somewhat high, but almost the same range of Nikon and Olympus in the same range, so see if you need a lens on this focal range (best for Safari, bird shots, shots from the top of the hill), then this is the one you should seriously consider. Other four third and bigger body lenses can be sometime fast but not as fast to focus and very heavy and very very expensive if you want this kind of clarity.
It is still probably the biggest in size among all Pany M4/3 lenses, but the IQ and features deserves the size, weight is very light comparatively.