i've got one. happy with it. just find it a bit big and heavy but overall, a fantastic lens. i brought it to hokkaido on my holiday last year, bashed around in the snow with it and the WR was added reassurance. constant F2.8 for that focal range is also very versatile.
i can't speak for other pple but the SDM on my copy is fine. i've had it for abt 1 yr and i use it often. if u're really spooked by it, get the extended warranty. $80 is cheap to cover a lens in that price range as to my knowledge, dead SDM may set u back by abt $300 for a copy thats no longer covered by warranty.
i've never used the new 18-135 but its using a newer DC motor and AF speed is supposed to be quite commendable as well. the AF speed of the 16-50 is ok with me - a few micro-seconds don't really bother me much - i'm not a very demanding user. my copy locks on relatively quickly and decisively.
This lens has been with me for over 3 years (and I wasn't the first owner), and is always with me when I go out shooting except for very specific birding or macro trips. Has survived hot, cold, wet and dry, and produces images that I consider excellent for a fast, wide-angle zoom. SDM failure? Happened to my 50-135, but not the 16-50 so far, it's still going strong. During the early days when the 16-50 was released, there were people complaining about poor optical alignment affecting the IQ, I believe this is something of the past already.
Never handled the 18-135 before, but from when I hear, the new DC motor is quite something. The SDM on the 16-50 is ok but not particularly fast, may not be a match for the DC motor.
DA*1650 is an excellent lens optically. But the SDM 'problem' does affect its reputation and sale. If you worry about SDM, then don't buy lah. There are always other alternatives out there.
As to AF speed, the newer DC 18-135 is definitely faster than DA*1650, quite a bit faster actually. But they are not in the same group when you comparing lenses. The 18-135 is more or less a good weather lens due to its smaller aperture, but since our Kr/K5 have pretty good high ISO performance, it works okay even indoor as long as the light isn't too dark.
If you wanna try both lenses, join out monthly outing. I'm sure you'd see a few of each in the group
I've been using my 16-50 for almost a year now. There's something special optically about the DA* lenses that don't translate into lab/imaging test results. It's crisp, and colours are great. Beats my old Sigma 18-50 2.8 hands down. Price-wise, it's kinda premium though. Some people will buy the Tamron 17-50 2.8, because you can get an FA 50 1.4 for the price difference.
I am very satisfied with my DA*16-50, as others have mentioned the color and contrast are very good. Its a bit soft wide open esp in the corners but sharpens up ALOT when you stop down. The price is rather high but you need to take the price of other similar lenses into context - the canon equivalent is about $1.5k, the Nikon equivalent is nearly $2k. I basically use it more than any of my other lenses, so it has definitely been good value for me.
I had the SDM in mine die but it was replaced under warranty, had no issues since then.
Sharpness wise quite about the same wide open, but da* gets sharper when stop down >f4. And da* produce more "contracy" n saturated colour, which is nicer
Size da* on the losing end.. Bigger n heavier.. Using 77mm thread filter..
By the way I own also the da*50-135. It is by far better than the two above when you need higher image quality. If you have more space for the focal length.. One of the best auto focus zoom I can say. Have compare it with my brother"s 5dm2 with 24-105L lens at f4 & f5.6 with same object at the same time, our da* never lose out to the L lens, but with less than half of the price..