This lens gets five stars if for no other reason than it has no competitors in the EF mount. However, it is a pricey lens designed for specialist applications and is not for most general purpose photographers.
Optics: Shooting wide-open (not really the intended use of this lens), the center is very sharp with the corners falling to good. Tilting or shifting results in even more drop-off in the corners. Shooting at working apertures (f/8 or smaller), the sharpness across the frame is excellent and there is no real vignetting and very, very little distortion. The only problem at working apertures is a bit of CA in the corners, but this cleans up very easily in post.
Ease of Use: Well-damped focus ring (as it should be for a MF only lens). Tilt and shift are easy to apply and lock, though the knobs are probably smaller than they should be. You should also note that it is possible to end up with one of the knobs under the viewfinder housing, which makes operation difficult, but this can be avoided and as is a common "feature" to all of the TS-E lenses. Lens cap is a bit better than the Canon standard and the lens hood is good quality plastic and locks on securely, unlike many Canon hoods.
These lenses are said to be "hard" to use, but with a liveview and a little knowledge, they are stupid simple to use. Just remember to use the DOF preview button to stop down the lens so you can see the final composition on the screen. It will take a bit before everything clicks, but you can master the ins and outs in an afternoon.
While there are many great resources on using tilt/shift lens, as a basic overview:
Shift allows you to change the perspective of the lens. If you shoot architecture and need buildings to have straight lines, this will be handy feature. You can of course accomplish the same task in most image manipulation programs, but you do sacrifice some of the frame to computer perspective correction.
Tilt allows you to change the field of focus from being perfectly parallel to the sensor. Thus, you can get near and far objects in perfect focus, which is an important ability for landscape or product photographers. Yes, you can get landscapes that are reasonably sharp from near to far by taking advantage of the hyperfocal focus distance, but this only puts near and far in "good-enough" focus. If you consider the cliched meadow in front of mountains photo, with hyperfocal focusing, you will find that the nearest flowers are annoyingly defocused and the mountains aren't as sharp as they could be. However, with a TS-E lens, you can avoid this and more or less have the entire scene in sharp focus.
Of course, the price you pay for this ability is a that you have to purchase a big, heavy, expensive, and manually focusing lens, meaning that this lens is not for everyone. However, if you want to shoot truly great landscapes (assuming you already have a good eye for things), then a TS-E lens is the way to go (unless of course you drag a LF camera out there, which is really the way to go). And if you shoot a lot of products, then this is really the way to go.
As a general note, tilt and shift are perpendicular to each other, so if you have the lens tilted up or down, you can only shift left and right. This is probably the behavior you want, but if you want to be able to tilt up and down while shifting up and down, you can either pay Canon to change the lens, or you can do it yourself with a screwdriver and a bit of care. The newest TS-E lenses allow you to do this yourself without tools, but for most users, this limitation will not be a problem.
Oh... build quality is also excellent. It feels like a tank and I have yet to have any problems with mine. I will of course keep everyone updated.
Also, these lenses mount the Canon series of extenders, so you get a great 60/4 and 90/5.6 as well if you already own the extenders. Optical quality drops off a bit with the 2x, but at working apertures, it is more than good enough for professional quality big prints.