I bought the Tamron this morning if anyone's interested and I'm exceedingly happy. I tried several lenses in store and felt this was the one for me!
EDIT. Another member has PMed me asking for some thoughts on the lens. Hopefully this'll serve to help him make a decision!
My local shop have a decent range of stock and allowed me to try a couple of lenses after I'd been considering the Tamron 70-300 VC USD or the Canon 70-300 IS USM (both F/4-5.6 and both with ultrasonic motor focussing).
I can't provide detailed technical information, but I can summarise the choices I made which led me to purchase the Tamron. My budget was ~£375.
I was looking for a lens which had sufficient focal length for sports (primarily rugby and motocross) and for wildlife. After reading tens of Amazon reviews, website reviews and checking sample images, I realised that focal lengths around 300mm (on a cropped body) would be sufficient. This ruled out Canon's 55-250, and their 70-200 L F/4 (no IS).
I then discovered that fast focusing would be of great benefit for my application. Having got a general feel of attitudes toward Canon's USM, I noticed that the Tamron equivalent lens was using the latest ring-type AF motor - the first lens of Tamron's to do so. When I compared (in-store) the Canon's incarnation of USM on its 70-300, it felt sluggish and noisy compared to the Tamron. It is also worth mentioning the Canon's tendancy to overshoot and return to point, whereas the Tamron will seek and stop dead on subject.
The Tamron's VC (vibration compensation) is its version of Canon's IS. Reviewers argue about many other points with this lens, but one constant is the VC. For me, and many others out there, VC is superior to IS which I would argue is a massive plus when handholding at 300mm. The real kicker with VC is that when you half depress that shutter, the image locks in the viewfinder temporarily, meaning camera shake is vastly reduced and your chances of a sharp image increased. I was having a look at it again this morning, and the gyro's effect is really noticeable - with finger off the shutter release, you're free to move the lens around to frame your shot - however, when half depressed to engage the AF, there's half a second where the image is locked (ideal time to take the shot) or for another second after, moving the lens around you'll notice a resistance through the viewfinder to the motion you provide. Overall, you can see a couple of my test shots in the lens gallery where there are a couple of 300mm examples. It's marketed as having 4-stop compensation, but I say that's wishful thinking, work to 2/3 and you'll be fine.
I'm no expert on bokeh, but the quality of the blur is well illustrated in these shots.
Personally, I'm now looking at moving into using filters and getting a wide angle lens for landscapes. Now that summer is here, I've learnt a bit about how CPL filters can aid your colour saturation, as long as they are positioned at the correct angle to the sun. It's also become apparent to me that some lenses rotate at the front when focusing - the Canon 70-300 IS USM does this, but the Tamron doesn't.
Another real world test is the weight, feel and build quality. The Tamron is significantly thicker than the Canon and heavier. I found the build quality of the Canon to be comparable to the 18-55 kit lens, and despite the zoom ring creeping as it was looser, there was a lock switch. The Canon also had a cool feature which I felt I would rarely use - a 2-mode IS where mode 1 enabled IS in horizontal and vertical and mode 2 enabled only vertical gyros to aid when making panning shots. The Tamron zoom ring is smooth and has a comforting level of resistance, while the focus ring can rotate infinitely and not damage the AF motor. This makes for a very nice opportunity to focus initially with the AF and if you're on the wrong subject, just modify it a bit.
The lens hood supplied shows absolutely zero vignetting.
I'm sure I've missed some things, but if anyone wants to ask something, then fire away and I'll do my best.
Some links that may help when researching;
I don't know how much credit EISA get, but it's been given an award as the best zoom lens in 2010-2011.
On a final note, I suggest you read through this thread in its entirety, replies and all. The Tamron is being compared to the Canon 70-200L IS F/4 - not even the 70-300 IS USM.