This will just be a quick review for now. Some of you may know, I bought a Sony SAL 18250 recently off ebay. As a used lens, I got it fairly cheap for £240. Suffice to say, I was rather chuffed
Sample photos will be available at http://cameralabs.com/forum/viewtopic.php?t=10401
1) Build quality
Compared to the SAL 1870, Tamron 55-200mm, this lens feels a lot sturdier. It sits tightly on the camera thanks to the metal lens mount. I really disliked the Tamron and SAL 1870's plastic one which I felt was flimsy. The zoom ring is stiff and does not drift when pointed up or down, and it is an internal focusing lens so the front lens element does not rotate. A polarizer is most definitely a good investment!
Compared to my trusty Sigma 28-200mm, the Sony is about 200g lighter. The lens hood also feels more flimsy. That said, I always felt that the Sigma was a little too heavy and off balanced the camera making it centre heavy. The SAL 18250 I feel is just perfectly weighted to balance out the A200. Your mileage might vary with other cameras that weigh different.
One thing that stands out about this lens is the focus. The focus is dead on and it nails the focus even in dark scenes that cause the other lenses to hunt. The Sigma was especially horrible at focusing in dark scenes. The SAL 1870 is slightly better but the SAL 18250 is a lot better.
In broad daylight, they're all fine so I can't really compare them.
This lens is really sharp at f/8. At 250mm, f/6.3 is slightly soft but this is quickly rectified by stopping down the f/8. At 18mm, f/3.5 produces very sharp images. Personally, I think a lot of the "sharpness" of this lens comes from the superb focus (see 2)). Unless you're comparing with a prime, this lens is definitely excellent.
Flare is well controlled. With the sun in the frame, there is no lens flare to be seen. Chromatic abberations are visible at 250mm, but they aren't too obvious at other ranges to be a problem.
There's some barrel distortion at 18mm but this is to be expected. If it really bugs you, buy PTLens and have that automatically correct any distortion (including CAs) in your photos.
The main reason for buying a lens like this is convenience. The ability to go from wide angle to telephoto in under a second opens up quite a few interesting possibilities when doing street photography and general people photography.
This is a light weight lens that produces very good photos at all focal ranges. If you want to take one lens with you, this is the lens to take.
5) What this lens is not?
This is not a prime. You get more flexibility in terms of zoom range but you sacrifice speed. With a maximum aperture of f/3.5, this lens is definitely not what you want to use indoors without a flash unless you like taking photos at ISO 1600 and above.
This of course means that my next purchase is going to be a fast prime. I'm eyeing a cheap Minolta 50mm f/1.7 or a Sigma 35mm f/1.4. Who needs drugs/cigarettes/alcohol when you're hooked on camera gear?