I'm considering buying a Tilt-Shift lens to fine-tune (read: add a layer of masochism) to my landscape excursions. I am interested in harvesting the accumulated experiences from this group. What should I seek out, and what should I avoid when buying or working with such lenses? Specifically, I have a Canon camera (a 5D) and am considering the brand's Tilt-Shift lenses. They make three kinds, the 24mm f/3.5 L, the 45mm f/2.8, and the 90mm f/2.8. They each cost about the same, around $1,000 at B&H. The 24 is an "L" lens, which may have something to do with weather-proofing, not sure. Any thoughts or comments?