So I picked it up yesterday in the store I'm working in and had the chance to quickly get a first impression of it today.
Keep in mind, that this is a superzoom which will inevitablely have to make compromises. I don't want to start a discussion on the subject if such a lens is "good" or "bad" - it has its clear purpose of serving as an allround lens for travelling when you just don't have the time to switch between several lenses all the time. It's also very light for those occasions where you dont want to pack yourself with multiple heavy lenses. If you need the best possible quality for a very special scene, you still will have to take a prime (or maybe a small-zoom-range) lens.
Basicly, when you pick up the lens, you feel that you've got a solid piece of glass in your hands. The AF, VC and Zoomlock switches are all reasonably placed so they can be reached well. The zoom has got some resistance to it to reduce lens creep (more on that in a moment). Between 70 and 120mm the resistance reaches its summit. However after a day of using it already starts to run a little smoother.
The front element is not rotating, which certainly is great news if you want to use a filter. Due to the wide angle, the included lens hood is naturally quite small, yet well shaped (bigger on the top and the bottom) to maximize its potential. The manual focus ring is a bit loose, but reasonably sized.
After so much praising you probably already suspect that there are some downsides as well, so here we go... The inner tube wobbles a little bit but it's not critical. Probably the biggest issue is lens creep - If you point the camera towards the ground and shake it, the lens will zoom in due to the weight of the inner tube(s). While I don't know exactly how big of an issue this is with other superzooms, I think it's still within reason on this one. As mentioned before, the zoom ring is pretty loose when switching to MF, so manual focusing is not that great.
Overall, Tamron has done a pretty good job with this lens, apart from the usual superzoom issues, such as lens creep.
I've read in various other comments about this lens (especially from those with Nikon mount) that the autofocus was very slow so I was quite worried when I did my first testshots. I discovered that in daylight, the autofocus is actually nothing to worry about - surely, it aint as fast as a USM Autofocus from Canon and if you want to professionally shoot sports, you should look for another lens - BUT for normal use it certainly is quick enough. In low light though, the story is a slightly diffrent one: The focus gets quite slow, taking about 1-2 secs if the subject is close and ocasionally it starts to hunt. At times it's actually unable to focus at all until you try a second or even third time.
Conclusion: The AF is just fine in good lighting conditions, whereas in low light it can get quite slow.
Not much to write about here... it works very fine - I was able to shoot at 1/60s at 270mm without problems and down to 1/8s at wider angles. I guess someone with more steady hands will even be able to take 1/4s shots without a tripod. You can tell when the stabilizer starts to work by its relatevely loud noise, it starts working when you half-press the shutter release button, and stops about 2 seconds after you stop pressing it again.
So the Vibration Compensation is probably the biggest improvement over the previous 18-250.
For optical Quality, check my thread in the lens galery
. To write a quick conclusion here (personal impression after 1 day of use): This lens is tack sharp in the center, still very sharp towards the edges, but starts to smooth out a little in the corners. CA's are nothing to worry about as far as I can tell at the moment, and vignetting is well controled.
The Tamron 18-270mm certainly is a compromise - but a good one! Optical quality is surprisingly good (at least as far as I can tell until now), and I'm especially pleased by the low amount of CA's. Build quality is ok, but not great - certainly good enough for a holiday. Vibration Compensation works great, which makes up a bit for the rather small max aperture. There are 3 bad points, 2 of which can be overlooked for an allround lens in my opinion: Probably the biggest issue is the slow and sometimes hunting AF in low light. typical for such a lens, it tends to creep when pointing up or down and lastly the aperture at 200< is quite small (f/6.3).
So overall I think I can highly recommend the Tamron 18-270 to anyone who's looking for a travel lens.