A colleague of mine, (a newbie in DSLR photography) needed some assistance to get the stuck filter on his EF-S 18-135 IS lens, so we went to the local Canon dealer (my office recently moved just 5 minutes walk from the Canon/Nikon/Tamron dealer ... me very happy) ... anyway, when we went to the store I was very surprised to find the Tamron 150-600 VC lens in display (apparently they've got 12 of them) ... usually in this country we receive new lenses a good 6 - 8 months after their release, so I was very pleasantly surprised to see this lens (also, this is an indication that this mighty Tammy is very popular even in this small country and that is the reason why the dealer here got them so quickly) ... anyway, the very customer friendly salesman (he's the one in the suit in the below pic) allowed me to play with the lens with his 5D MK III (unfortunately I was not carrying my camera, as it was meant to be a short visit to get the stuck filter off the lens ... so I don't have any sample images to compare).
Having played with the lens for about 10 minutes, these are my first impressions of Tamron 150-600 VC lens:
1. The AF, accuracy & speed are very very good (just as good as the EF 100-400 L IS)
2. Build quality is very poor in comparison to Canon EF 100-400 L IS (I don't think it will survive a fall) ... but considering its very low price and superb performance, I don't see how anyone can improve the build quality at that price point.
Unfortunately, the Tamron 150-600 VC lens price here (as usual) is very high i.e. US$ 1863 ... obviously I would not want to spend that kind of money on Tamron (despite its superb image quality, bcoz I am afraid that it may not last long due to its poor build quality) ... but ordering from USA at around US$ 1220 (1069+customs+shipping) sounds reasonable ... I was really excited about it for the past month or so, but after having held it in my hands, now I am in two minds
... my Sigma 150-500 OS has much better build quality then this Tammy and I know from my personal experience that lenses this big tend to get bumped into things very easily, so not sure if I want to spend US$ 1220
Anyway, here is an image made at the store with the Tamron 150-600 VC & Canon EF 100-400 L IS, next to each other ... compared to the Tammy the build quality of Canon is far superior.
PS. Image below is made with a mobile phone, hence the great quality