I just sold my Canon 70-200mm 2.8 to B&H and picked up the 40-150mm 2.8 Pro. A few reviews that really solidified the decision for me were this one by Robin Wong and then this one with the extender (didn't get that yet). This photographer also used it for an airshow with great results and has another gallery with some killer Osprey shots. You'll also want to check out the incredible work by Jay Dickman who is a long time micro 4/3 user.
10 immediate impressions:
1) Its weight is substantial but FAR less than my old Canon (about a full pound actually). It paired very nicely with the EM1, and I did not feel the need for any additional grip.
2) It feels solid - just like the 12-40mm. The weatherproofing will come in very handy.
3) The built in lens hood is a very cool design and for the first time ever, I may actually use the lens hood (especially to keep rain off the front of the lens).
4) Insanely good macro capability.
5) Nothing physically moves in or out when you change the focal range. It's all internal which is sweet!
6) Beautiful bokeh at f2.8 means the out of focus areas are just lovely washes of color.
7) Tack sharp - even wide open.
8) Very fast to focus.
9) Effective focal range of 80-300mm makes it very versatile, and can be used for wildlife.
10) Constant aperture of f2.8 through the focal range will come in very handy for low light work, or if I should pair it with the 1.4x extender.
Here are some images from my first day out with it.