I have been following the Tamron super zoom lens for some time as I had wanted to buy one as well. But I later settled for the 24-135SP which I felt had better glass although was a lot wider(in diameter) and heavier than the 28-200XR. These were my considerations about 1.5 years ago before I went for a 2 week holiday in Britain:
IMO, the 28-200XR has the best glass of the 3 (28-200 II, 28-200 XR, 28-300XR) lens and the most compact form factor. It's also internal focusing so u can use any filter u want without readjusting. In addition, it has the benefit of being long enough but not so long that u need to pack a tripod. Outdoors, u'll easily get 1/250 at max aperture of f/5.6. So u can hand hold easily and still get sharp shots. The wide end is only moderately wide, good enough if u dun need to take wide expanses. Chromatic abberations are invisible at 4R or even 8x12 (saw a review at photo.de if I remember correctly). The XR version has the smallest and lightest form factor - compared to the 28-200 (1st gen) and the 28-200 II. Also has a moderate macro at the long end to boot. Quite worth.
However, I decided against the 28-200XR as I preferred to have a wider end. At that time the 24-135SP was launched. It has good glass, focuses quite fast even on a slow 505si and there was little distortion all round at both ends. Pictures were sharp mostly but can be a little soft at the long end. But u have got to be really looking for it. The only problem I faced was when I stacked a circular polariser on top of a UV filter, the two filters caused the lens to vignette at the wide end (widest 24mm). However, the vignetting was minimal. Usually on the top two corners had a little blue smudge. To the untrained eye, it was not obvious at all. But of course ayone who frequents ClubSnap should be able to spot it a mile off. It served me superbly on my trip and it is my usual one-size-fits-all lens that I carry around these days. The wide end is really quite good since u can really capture quite a lot especially when u are constrained by space. I was taking Picaddily Circus and could only stand at the little pavement about < 2m from my subject. The 24mm end could swallow enough of the curvature of the buildings ringing the Circus so that the shot was quite dramatic. If the wide end had been 28mm, it would not have been such a good shot! Paid $580 for the 24-135SP new. U might get it cheaper now tho'. Hardly any on the 2nd hand market.
Hope this helps!
