Alas, I lost money on the f/2.8 II because I bought it at one of its highest prices in years - no rebates or anything at the time. I needed the lens for a shoot when I bought it so technically it paid for itself, but I have sold every other Canon lens for the price I paid or more. Yes I have plenty left over to cover the f/4 IS, but I don't like to lose money, even if it's just in principle vs. reality. A 3 year rental would have cost me how much?
On the sunstars - I have been shooting with the 24-70 f/2.8 II for a while now and don't find the 9-bladed aperture to be an issue even at 70mm. I'm not sure I have a strong preference either way - sometimes the spiky 9-bladed sunstars are a nice effect - like for nightime cityscapes, and sometimes they are odd and distracting, but sometimes the softer 8-bladed sunstars just look odd and smeared. Obviously the aperture makes a big difference, too, so there is some control. And if you think about it, at the 16-35mm focal length, the sun is never going to be very big in the frame so I don't think it will be much of an issue.
Also, regarding Tom's point about astrophotography, the f/2.8 II like all older Canon zooms and most primes has terrible coma in the corners. The 24-70 f/2.8 II does not and it's possible that the f/4 IS won't either. Combine that with the 5DIII or 1DX and the ISO can make up for the aperture, at least in good conditions.
ejenner, thanks for sharing your thoughts on the lens and how it compares to the TS-E 17. I looked at the distortion charts on TDP and the 17mm seems much less distorted, at least in the neutral position. If the f/4 IS is as sharp as the 17mm, I'll be a very happy man
I've lost money on the 3 lenses I've bought and sold: about 200 on the 17-55, about 50 on the 10-22 and about 150 on the 16-35 II. The one that really surprised me was how soft the market the 16-35 II had become. When I bought it, it sold used for 1300, but I got mine for about 1200. I sold it for close to that, but the loss is mainly from fees. The only auctions that I saw go for much more than 1200 were usually scammers with no feedback scores.
The one stop difference compared to the 16-35 II won't matter as much to me or those that have faster primes. With f/1.4 options at 24 and 35, you lose on focal length versatility, but the two stops is more signifcant than the 1 stop difference compared to the 16-35II.
It's true that IS is not as effective at shorter FLs, but I'd still rather have it than not. It helps when the shutter speed gets as slow as 1/2 second, which is handy for travel (i.e. museums, blurring water flow minimally). It looks like it'll be an ideal travel lens. I love the idea of a 16-35 III that is as good if not better optically than the 16-35 f/4 IS, but that is not a option now.