Pros: Versatile wide range, very portable, great quality.
Cons: No lens hood.
I think a lens system can be judged by its coverage of the shorter focal lengths (wide angle). Manufacturers are quick to cover the medium to longer telephoto ranges, but in many ways, those are easier focal lengths to build than the wides.
Why shoot wide?
A common misconception is that wides are mainly landscape lenses. More often, medium range range lenses are used for landscapes - wide angle from a long distance is just, well, too wide - and the viewer will not be able to see much detail.
Wide lenses, however, are well suited for shooting in cramped conditions - think interiors crowds, and dense cityscapes. This is what you want a wide for. Another property of wides is that they exaggerate the space between foreground and background subjects, which can be used for interesting effects, and is also the reason why people don't shoot protraits with wides (this effect distorts faces).
Overall, I love shooting wide. Wide provides the context in which the subject exists. It is a necessary component of a photo's narrative - without it, you have no story, merely portraits....
The Olympus 9-18
The Olympus 9-18mm is a 18-36 35mm equivalent zoom lens priced within the enthusiasts range of around $500. For my type of shooting, it is near ideal - I can grab most interiors in a single shot, yet on the streets, a 36 mm reach is still enough to frame fairly closely around subjects. Actually great for moving subjects on the street, as wide gives you more room to work with as your subject sweeps across the lens.
The Olympus 9-18 validates the Micro Four Thirds architecture as a whole. Let me explain: the micro four thirds design philosophy was that a slightly smaller sensor size from the typical DSLR would dramatically reduce the size and weight of the lenses with minimal compromise on image quality. The 9-18 is a case in point: the lens is mere 155 grams and 49mm in length - over half as light as a comparable DSLR setup. I walk around with the lens in my coat pocket all day - no problems. Try doing that with a DSLR equivalent (hint, you won't).
The lens body is another collapsible design by Olympus, but unlike their horribly loose 14-44mm kit, this lens feels wonderfully tight and vibration free. The manual focus ring has just the right amount of resistance, and the hard plastic body feels great in the hand. Really, this is a lens that inspires confidence. The lens is threaded to take 52mm filters.
There is a tad bit of barrel roll distortion that is easily removed in software, though I confess to liking the effect. Likewise, chromatic distortions are present, but easily tamed. I've seen other reviewers refer to this as a sharp lens - I wouldn't say it was particularly sharp, but certainly not a soft lens by any stretch of the imagination. You don't need me to tell you that f4 - 5.6 isn't a bright lens - great for outdoor daylight, good for well lit interiors - and not the lens to reach for when you want BOKEH (whatever that is). Still, f4 - f5.6 aperature range is a good concession for the afforadble price. If you own an Olympus M43 body, the built in image stabilization makes this lens that more versatile in lower light conditions - but panasonic camera owners should be aware that like all Olympus lenses, this lens is not optically stabilized.
AF speeds are fine, though I can only judge on my e-pl1 which itself is much slower than the latest e-p3 models from Olympus. The motors are quiet.
One minor issue: the lens hood is sold separately. Come on, Olympus...
Overall this is a great lens at a reasonable price (around $550 street). It is very portable, a solid performer, in a versatile focal range - in short, this lens delivers on the promise of the micro four thirds design. Easily my favorite lens in my kit bag.