Let's start with the downsides:
Manual focus. It's not really a huge deal, but it would be nice to have it for when the lens is neither tilted nor shifted, or even just to have contrast detection.
Aperture. It's not f/2.8. I don't expect it to be that fast, but it's still technically a downside.
Image quality. I have serious issues with the files in Lightroom. From 90 feet away at 3:1 magnification I cannot read facial expressions. Even my 200mm f/2.8 can do it for half the price. Jokes aside, I can read the facial expressions at 60ft/2:1 mag. That's how sharp this lens is. I was impressed with the image quality of the lens when I first looked at files in LR, then I realized I was looking at them with 2:1 magnification. I was even more impressed! The resolution of this lens is simply astounding. Pixel peepers can finally get a wide angle and rest easy. The optical quality of this lens matches my 300 2.8L IS. It's that impressive. The files hold up to massive scrutiny.
Mechanics. They worked flawlessly. Nothing special to report here. IQ took a minor hit from maximum shift or tilt as to be expected.
My suggestion? If you have your general purpose lenses down (16-35/24-70/70-200 or primes), buy this lens. The optics are amazing, and it's a useful focal length. I sold mine due to the type of photography I do (low light, portraits, wildlife), but I would buy it back in a heartbeat if I had money to burn.