On an APS-C camera there's not a huge difference between the optical performance of the Tamron and Canon lenses. On FF the Tamron is better at the edges and corners of the frame, especially full frame when at the telephoto end of the range.

What's interesting is that Tamron seem to have developed a ring type USM motor for AF that's very similar to Canon's ring USM, with the same full time manual focus capability. This is the first Tamron lens to use such an AF drive system and I expect that they will include it in future lenses. It seems to work well, just like Canon's ring type USM drive.

The factors that make the Tamron a good buy are the included lens hood, 6-year warranty and ring type USM motor. The unknown is reliability and compatibility. Those factors will only come into play after a few years. In the past Tamron lenses have had a pretty good record of compatibility with new Canon bodies though, so I don't anticipate any surprises - but you never know for sure.

The angle of view numbers are stright from Tamron, so I assume they refer to full frame usage, but I'll check (and add in the APS-C numbers).

Indeed. The EF 70-300/4-5.6L IS USM lens has both of course, but at $1499 (after $100 instant rebate)  it should have - and possibly be gold plated as well. I have a request in to look at one, but I don't know how soon that will happen. It will have to be unbelievably good to be worth $1499 - which is the same the EF 100-400/4.5-5.6L IS USM sells for, and even better to sell for $1599 after the current rebate expires at the end of the month.