The Tamron SP 24-70mm f/2.8 Di VC USD is a new 2.9x zoom that was announced April 2012 and is the first ever stabilized large-aperture standard-zoom compatible with full-frame DSLRs. With its gold-ring, constant f2.8 aperture, and a price-tag of around 1000 EUR it is in direct/fierce competition to Nikon’s own professional AF-S 24-70/2.8G Nikkor or Canon’s EF 24-70mm 2.8 II L USM. This makes it a very interesting lens that is indeed one-of-a-kind.
You could of course also use the Tamron 24-70mm on a APS-C/DX body where it gives you an equivalent 36-105mm coverage plus future-proofing should you upgrade to a full-frame/FX-body in the future. Complement this lens with any APS-C/DX 10-24mm wide-angle zoom and you can seamlessly cover a 7x zoom-range from ultra-wide-angle to short tele with only two lenses.
The Tamron 24-70mm f/2.8 VC is available in Nikon, Canon-, and Sony-mount, although the latter does not offer vibration control (VC) as Sony-bodies sport body-based image-stabilization; that said the Sony-variant without VC is no cheaper than the optically stabilized versions for the other two mounts. In this review I’ll put Tamron’s SP 24-70mm f/2.8 Di VC USD zoom to the test on a Nikon D800 to see whether the lens is a good match for the highest resolution 36 Megapixel DSLR.
Facts from the catalog
As usual I’ll have a look at the technical data first. I’ve rated the features with a [+] (or [++]), when it’s better than average or even state of the art, a  if it’s standard or just average, and [-] if there’s a disadvantage.
Size (diam. x length): 88 x 109 mm (3.5 x 4.3 in). It’s a little thicker (+5mm) but substantially shorter (-24mm) than Nikon’s AF-S 24-70/2.8G and thus looks less conspicuous – when zoomed-out to 24mm. But zooming in adds another 30mm to the length of the lens and at 70mm it is not much shorter than Nikon’s. 
Weight: 825g (29oz) vs. 900g of the Nikon 24-70/2.8G or 805g of the Canon. Together with a full frame body you’ll be schlepping 1.7kg around. 
Optics: 17 elements in 12 groups. Twelve groups have 24 air/glass-surfaces that produce a lot of opportunities for flares and ghosts. We’ll see how this works out in praxis. The cross-section shows a lot of special elements: four aspherical and two low-dispersion and 2 extra-refractive elements. [+]
Closest focus distance/max. magnification: 0.38 m (15 in.) / 1:5. In my test I was able to go down to 1:4.6. This is not bad for capturing nature close-up – see one of my images from the gallery. [+]
Filter-thread: 82mm = as big as what Canon’s needs, Nikon’s lens works with the more standard 77mm. 
IS: Yes = helps a lot, Nikon and Canon are no match in this respect. [+]
AF: USD (ultra sonic drive), does work on D60/3×00/5×00-Nikons or similar drive-less bodies from Canon, and there’s manual-focus override by simply turning the focus ring [+]
Covers full frame/FX or smaller = very good [+]
Price: around 1000 EUR new (incl. 19% VAT) = reasonable. The alternatives from Nikon, Sony and Canon are 50-100% more expensive. [+]
Lens-shade is included and reversible for transport, the front lens-cap is of the pincer-type and the rear lens-cap can be mounted in each the the three possible orientations (unlike Sigma’s). But there’s no lens pouch. 
Distance information is relayed to the camera, so the Nikon body can do all the advanced exposure-related stuff with this lens. But this is true for all alternatives too. [+]
Aperture ring = no, just like all competitors. 
Sealing: yes! A rubber grommet at the lens-mount, like with the Nikon or Canon. [+]
The score in the “features-department” is 0[-]/5/8[+]. All-in-all the lens ticks all important boxes. And the missing lens-pouch and the larger than usual filter thread are easily compensated for by the relatively low price.
A stabilized 2.9x zoom with a focal range of 24-70mm may be your best choice for a full-frame/FX-body when you want to be prepared for many standard situations but want a larger maximum aperture and better image quality than you would expect from kit-zooms. It is arguably the best zoom range and focal ratio for wedding and portrait photographers.
At 24mm shortest focal length it lets you capture a crowd in tighter spaces or shoot architecture. And the 70mm on the long end gives you some reach and working distance for classic portraits and street-photography although you may find it too short on a full-frame/FX-body in some situations. It has a one stop larger aperture than kit-zooms and its image stabilization should add another 2-4 stops over a non-stabilized alternative. That gives you quite an advantage of hand-holding power under dim or fading light. Be aware though that image-stabilization is no advantage for fast-moving subjects like children or sports. In the latter shooting situations you should crank-up the ISO to get faster shutter-times.
– The Nikon AF-S 24-70mm f2.8G ED: Not stabilized and at 1500 EUR it’s 50% more expensive. This pro-lens from Nikon claims better performance. But only a direct shootout can tell.
– From Sigma there’s the AF 24-70mm f2.8 EX DG HSM. It’s the cheapest alternative at around 750EUR. Non-stabilized like all the others and 15mm shorter than the Tamron with the focus ring in front and the zoom-ring near the camera. Tests showed it to have pretty poor border/corner-performance wide open.
– For Canon users there’s the Canon EF 24-70mm f2.8 II L USM at around 2200 EUR. The most expensive lens of the group, but still not stabilized.
– And Sony users have the Sony AF 24-70mm 2.8 (what, no fancy acronyms?), at around 1800 EUR it’s the second most expensive lens. Stabilization is granted through the body-based image-stabilization of Sony DSLRs.