I quite often use my 90 ts-e lens and you might reasonably wonder how important is it that one purchase a tilting lens. Perhaps you don't even know how they work but are curious.
Canon and Nikon and Hartblei sell lenses that tilt or tilt and shift. Many people are under the impression that lens tilt increass depth of field. This is incorrect. Tilting a lens does not add any depth of field at all, it just changes the plane of focus. In a normal photographic situation, when you focus a lens at 10 feet, there is a spot 10 feet directly in front of the lens which is sharp, and usually the area of sharpness extends left, right, up and down perpendicular (90 degrees) to the line from the lens to the centre of the image focussed at 10 feet. This invisible wall is usually flat but sometimes curves nearer at the edges as a flaw often seen in wide angle lenses (my 17-40 certainly has a curved plane of focus, focussing nearer at the edges and corners of the image. Anything in front of the plane of focus (that invisible wall) is progressively more and more out of focus the further from the "wall". The same thing happens behind the wall (ie. further away).
It is possible however to take that invisible wall, the plane of sharp focus, and tilt it in any direction we want. If the thing we are photographing is relatively two dimensional, ie. all in the same plane, it should be possible to shift the plane of focus to match the plane within which the subject lies.
So, for example, if in the foreground we have some short plants, in the middle distance some medium height flowers and in the background some tall ones, the flowers (but not the bases of the plants) lie more or less in a single plane from near low to far high, and it would sometimes be helpful to change the plane of focus to match the position of the flowers. This is what happens when we tilt the lens.
If I tilt the lens downwards, it is at an angle to the sensor or film instead of being parallel to it. The distance from centre of lens to top of sensor is now larger, the distance from centre of lens to bottom of sensor is shorter. As shorter distsances focus further and longer distances between lens and sensor focus nearer (think extension tubes to focus close), you now have a situation in which the top of the sensor focuses far, the bottom near. Remember of course that the image is upside down on the sensor so what is really happening is that the low foreground is focused sharply on the top of the sensor, the far high background flowers are sharp on the bottom of the sensor. Since the middle distance flowers are also on the same tilted plane, they too are sharp.
What isn't sharp is anything that lies above or below this tilted plane - so the bases of the plants are blurred, and any plant that is exceptionally tall compared to the others will also stick up out of the plane of focus and the stem will be sharp but neither base nor flower will be in focus. At wide apertures this can look quite bizarre, but with a small f stop, the unimportant parts are sufficiently sharp that the odd plane of focus isn't problematic.
In table top photography especially, it can be very effective using wider apertures and a tilted plane of focus, sometimes keeping the plane vertical but tilting it from parallel with the sensor so it is near on one side and far on the other.
When photographing landscapes with a tilting lens, the amount of tilt needed to shift the plane of focus from vertical to nearly horizontal can be very small, fractions of a degree. On the other hand, in close up photography, you can run out of tilt quite easily.
Focusing into the corners of an SLR image has always been problematic - at least until live view with magnification came into place. The ability to place the area of magnification anywhere in the image means that you can check absolutely that the plane of focus is exactly where you want, and at least on my Canon 5D2, you can stop down and confirm that the depth of field on either side of the tilted plane of focus is sufficient to handle any important part of the subject which projects above or below the tilted plane.
With my 90 ts-e lens, the tilt does not exactly centre on the sensor and so there is some shift in the image as you tilt, which will require reframing the image (or using the shift). I usually just do a minor reframing.
Given that focus blending with Helicon Focus does in fact result in greater depth of field and copes beautifully with three dimensional subjects (that don't lie in one plane) the obvious question is - so what's the point of a tilting lens?
The advantages are several and may or may not be important to you.
1) there is some loss of resolution in focus blending - not a lot, but perhaps enough that if a tilt will do the job, then it may be preferable. Mind you since any lens can focus blend...
2) you can stitch with tilting for higher resolution images where this would take a huge number of images in a combination focus blend stitch.
3) With tilting, you see what you are going to get.
4) Tilting can be used with shallow depth of field to blur areas of the image. This can be done to some degree with focus blending but in different ways.
I quite often use the shift ability of my 90 mm. lens to do a stitch, shifting the lens one direction and the body by and equal amount in the opposite direction to capture a series of three images (left, right and centre) to make a square image which would otherwise have required a crop - so instead of a 3000X3000 pixel image, I get a 5000X5000, which does make a significant difference, to wit prints that are 67% longer on a side. This is the difference between a 10X10 print and a 16X16 print.
I don't tend to use the shift capability for correcting perspective very often, preferring to correct perspective in Photoshop, though a recent demonstration of loss of resolution in the stretched part of the image was impressive - at least in theory - in practice, I have not noticed substantial quality issues in my usual print sizes (13X19) even when stretching a 17 mm. image (ie. severely leaning backwards).
I like my 90 for its close focusing and high resolution, but I probably wouldn't rush to buy it again, now that I'm happily focus blending. Then again, I sure wouldn't mind having one of those new 24 ts-e II lenses - so who knows. not going to pay $2500 for one any time soon.