"Mine is smaller than yours!".
You will rarely say this as an advantage especially from the macho male photographer point of view. But in the case of the Olympus M. Zuiko 40-150mm F2.8 PRO the story can be true considering the extended versatility and the reduce size of this model compare to the DSLR counterparts. Furthermore the same statement can be proclaimed for all the M4/3 format equipment.
The Olympus 40-150mm F2.8 PRO stays a massive lens by M4/3 compact standards. But it replace two DSLR Pro lenses, the traditional 70-200mm and 300mm both with maximum aperture of F2.8. So the trade-off is still at the advantage of the Olympus Pro telephoto zoom lens model. It prevent also the necessity of changing lenses or to operate two different cameras.
Using big telephoto zoom lenses increase the unbalance tendency between the optic and the camera. That fact can be critical if you need a correct handling especially if you are panning the action not only for the effect but simply to follow the subject. Fatigue can be a highly distractive factor that will affect your ability and motivation to produce pictures. You can enhance your handling by adding an additional grip or vertical power grip on the camera. If you adopt a more static position a fix support like a monopod or a tripod will be a great help for stabilisation and a more careful picture cropping. In fact the nature of your subject will characterize your working methodology.
As a Pro lens the Olympus 40-150mm F2.8 is getting a superior quality of construction. The zoom and focusing rings are larger and will turn nicely with a simulated friction similar to old-fashioned lenses. The click-on manual option available on the focus ring is a very secure and fast way of selecting between auto or manual possibilities. By selecting the manual focusing only position you over rule the autofocus fonction and get a better focusing ring resistance similar to the previous manual focusing lenses. It replicate what is already present to the Olympus 12-40mm F2.8 Pro lens which is often the companion optic chosen by many. As for all the Olympus other M4/3lenses, no aperture ring are offer on that model assuming that it will done by one of the dials of the camera.
The Olympus M. Zuiko 40-150mm F2.8 Pro lens is a very fine, constant, versatile, workhorse optic that is reliable into various conditions of uses. It is part of the traditional duo along with the Olympus M. Zuiko 12-40mm F2.8 Pro and also with the T(h)ree family if combined with the Olympus 7-14mm Pro. All these pro lenses will form a very competent equipment when teamed with OM-D bodies like the E-M1 and E-M5 or even the E-M10 (Original or Mark II versions of any of those models).
Olympus is very serious about its Pro line of lenses involvement and offer a complete line-up of fine, fast and sturdy optics with the M. Zuiko 7-14mm F2.8, 12-40mm F2.8, 12-100mm F4.0 IS, 40-150mm f2.8, Fisheye 8mm F1.8, 25mm F1.2 and 300mm F4.0 IS. Combined with the OM-D E-M1 (original or Mark II), an independent professional photographer will find a very competent and compact eco-system that will sustain most of its need.