In my personal opinion, people complaining about lens size and weight are a joke. If I'm going somewhere where I need to image quality increase from my iPhone's camera to my DSLR, I don't care if I have to lug around a 10 pound lens or two as long as my images are of the utmost perfect quality. Granted, I'm young and in good shape so I don't have issues carrying stuff all day. Still, I wish manufacturers would keep the weight savings stuff to the consumer lenses and just let the pro lenses be god awful heavy if it means better image quality, and many of my "tog" friends agree with me in this area.
That was off-topic, but now on topic: this new lens does look very nice. I've always been a fan of Sigma's nicer lenses, they are great price/performance, especially used, and they offer some lenses that nobody else does. Tamron had some pretty awesome lenses before, such as the 28-105 f/2.8... I look forward to these third party manufacturers continuing to build high quality glass as it'll either force camera-brand lenses to come down in price or go up in quality... always good for us (consumers).
150-600... wow and f/6.3 while not great is still very respectable at that kind of length. If the quality is even half decent this could be great, though I'm not sure quite what market they're shooting for.