> Is it hard to learn to use the tilt/shift lens ...
I think most would feel comfortable with these fairly quickly. Once you have the movements, you will not want to give them up. I have the 90 and 45, and would love to have a *good* 24mm tilt-shift.
The tilt shift mount forces the rear element to be even further away from the film/sensor than the does the usual SLR mirror. So, even the 45mm TS-E is a retrofocus design, and the optical performance shows it.
Additionally, a large image circle (58mm, I believe) is needed to cover the frame when the image is shifted off axis.
The bottom line is that designing wide angle tilt-shift lenses is tough -- much more of a challenge than even the normal SLR wide angles.
On the other hand, the 90 is about like a medium format "normal" lens, which have been very good for years (as any Rollei twin lens owner will attest). The 90m TS is the best optically because it's not a retrofocus design.
I've found the 45mm TS to be quite useful in the past -- usually using it at f/11 to minimize its performance problems. However, whether it'll be able to make 16x20 prints with the 5d2 that have the fine details I like remains an open question.
The old 24 TS was, as noted previously, a dog if display size prints were the target. Whether the new one will be able to overcome the optical challenges is an open question. The design looks more like the famous Zeiss 21mm Distagon than any wide angle I've seen from Canon. If the results are as good at f/8, it'll be tempting. But, as also mentioned previously, tilt is much less needed at 24mm due to the depth of field. I used a 45mm with my medium format Bronica rangefinder and found 2 focus zones could capture most of what a tilted lens would capture.
Note also that retrofocus lenses tend to be sensitive to focus distance. That is, their performance falls off significantly when they are used at other than their design distance. To offset this problem, many wide angle lenses use floating elements. But, with a tilt, even floating elements can't make the lens optimized for all the different distances. So, I'm skeptical even of the new $2.5K Canon 24 and 17mm TS lenses will be able to make top notch display size prints.
Additionally, with Photoshop we have tools to deal with perspective. So, the need for a shift is less, in my view.
Still, wide angle tilt-shift lenses are a ball to use. The old 35 TS for the Canon FD was one of my favorite lenses prior to my decision to focus on B&W display. So, if Canon can pull it off, I might well be a customer.
Note that the MTF curves Canon has provided at http://www.usa.canon.com/consumer/controller?act=ModelInfoAct&fcategoryid=156&modelid=18175 presumably show f/8 performance at 10 and 30 lp/mm. The performance dips at about 10 mm out to a rather low level -- about like the Canon 24mm f/2.8 at the edge of the frame. On the other hand, the lens improves past this point (weird curve, probably due to the aspheric front element). The average might be good enough for serious work. It's close. It'll take more than this chart to convince me.
Paul
www.PaulRoark.com
Frankly, I expect to see dedicated wide angle digital rangefinders.