The following is not a super technical sort of review. If you're looking for a detailed rundown of technical specs and photos of the lens itself, take a look at Ken Rockwell's in-depth review. Instead, this brief overview takes a look at the benefits of using the Fujinon XF 18-135mm zoom in everyday photography. It retails regularly around US $900, but you can periodically find it on sale for $700. I have no affiliation with Fujifilm, apart from being a happy consumer of their gear.
Zoom lenses have historically not been favored by professional photographers. They are typically not as fast as prime (fixed focal length) lenses, which means they're generally not well-suited for low light situations. They also tend to be less sharp than primes, and the bokeh (out-of-focus area in a photo) quality doesn't usually match the creamy look of high-end portrait lenses.
That being said, zooms are no longer merely second-rate, inexpensive lenses targeted towards amateurs. Part of the reason for this change is that the quality of glass produced by the big name camera brands has improved greatly. (Third party lens manufacturers are still a mixed bag.) Another factor is the introduction of smart technology in newer cameras that compensates for the unsteady hands of the photographer. This feature compensates for the slower speeds of the typical zoom lens. Very high end zooms feature relatively fast, fixed apertures, but you'll pay a hefty premium for lenses in this category.
Four months ago, I bit the bullet and purchased the Fujinon XF 18-135mm f/3.5-5.6 R LM OIS WR lens. It turned out to be a good decision. I wanted a zoom with more reach than the excellent XF 18-55mm kit lens that came with my Fujifilm X-E2, without having to shell out a small fortune. Despite owning a handful of excellent lenses from the Fujifilm lineup, the 18-135mm is the one that is now most often found on my camera body. Admittedly, it's not as fast the super sharp 23mm f/1.4, and the bokeh isn't as delicious as what I can get with my 60mm f/2.8 macro lens.
What this lens offers that the others don't is versatility. With an effective reach of about 28-200mm (in terms of 35mm equivalence), it covers almost any application I might need. If you shoot sports or wildlife, you may need a longer reach. For most anything else, this range covers better than 90% of the typical subjects I photograph. I would have absolute confidence taking this lens alone on the trip of a lifetime. (I might also want to pack the 23mm f/1.4 lens for low light situations, but it wouldn't be the end of the world if I left it at home.)
Equally importantly, the lens boasts an insane 5-stop optical image stabilization (OIS). Thanks to OIS, I can shoot at ridiculously slow shutter speeds, handheld, and rarely miss a shot due to slight camera movement. When it can nail the focus, my XF 60mm is definitely a superior portrait lens. The critical part of that statement is when it can nail the focus. The 60mm has no image stabilization, so even when the lens finally locks on focus, any slight movement on your part can easily result in a blurred exposure. With the XF 18-135mm, I can easily take handheld shots down to 1/15 of a second or even slower - something that my non-stabilized primes have trouble pulling off.
Personally, I'd rather use a lens that may not quite attain optical perfection but proves itself to be reliable and consistent in a broad variety of lighting conditions. It also doubles nicely as a near-macro lens; you can zoom in from as close as 1.48 feet for great detail. In brief, the 18-135mm delivers.
Most reviews of this lens include a caveat to the effect that "of course" you won't get results equivalent to a prime, but my experience so far suggests that this assertion really depends on the kind of photography you plan to do. If you shoot portraiture exclusively, this lens may not be your first choice in the studio. If you're like me, however, and shoot a wide variety of subject matter, in all kinds of lighting conditions, it's hard to find a better lens that does what this one can do for you.