For my m43 outfit I try to keep things small. I also have full frame, so the m43 stuff has to be compact to make sense. My main m43 telephoto has been the Panasonic 35-100 f/2.8. It's terrific and quite small, but sometimes I want a bit more reach, so I bought the Olympus 40-150 f/4. Both lenses are sharp even wide open. But since I'm not going to carry both at the same time, I have to decide if I want the extra reach of the 40-150 or the extra speed of the 35-100. You can't go wrong with either, and they both seem to work equally well on Olympus cameras. Neither of these lenses is cheap, but as I've said before, with a small sensor like m43, you really can't cut corners with lens quality.
The 40-150 f/4 is a "Pro" series lens which means two things. it's water resistant and it's really sharp. Otherwise it's like other Olympus lenses, partially metal and partially plastic. I wouldn't say the zooming is silky smooth, but it does the job. The lens collapses to reduce length when not in use. This works well because there is no locking mechanism--you just turn the ring. Bravo, Olympus! I've had collapsable lenses with lock buttons and they are a two-handed, time wastin' PITA.
The included lenshood does have a lock button, but since they got the lens right, I can forgive the hood. (Locking lenshoods make sense if the hood is rectangular; the lock assures proper positioning of the rectangle so it doesn't cut the corners of the image. With a round hood, there is no point). I just have to remember which of my lenshoods need to be unlocked and which don't, so I don't break anything.
Summary. Sharpness is very good, focusing speed is very good, image stabilization is very good. Contrast seems a little low, but ok. Focuses really close! Personally, I don't miss having extra buttons on the lens or the manual focus clutch thing. I have lenses with these features and don't use them. Your needs may be different. One interesting bit of trivia; the 40-150 always opens to f/5.6 for starters. You would think it would default to f/4 but apparently not.
All in all, a very good lens priced like a very good lens. If you're happy with a less expensive zoom in this focal length range, don't try the 40-150/4 because you'll be spoiled.