I started out with MF, as that was all we had back in the 70's. There is a significant difference between using a good MF lens, and using an AF lens in MF mode. This is simply because the demands placed on the focusing systems vary considerably between AF and MF. To make a fast focusing AF lens you need a relatively short throw on the focus movement, this makes it quick for the motor to move from MFD to Infinity and back. Also the motor drive can be very accurate while moving over very short distances. In comparison an MF lens needs a long throw, so that there is significant movement available, as use humans are not nearly as accurate in moving and seeing the difference in the focus position required. Thats not to say that one cannot successfully MF with an AF lens, or that a long throw AF system wouldn't work, but both will be somewhat sub-optimal.
There is one other thing to consider when looking at MF lenses in an EF mount. That is aperture operation. In adapted lenses that is always also going to be completely manual, so you will have to do the final composition etc at the taking aperture. Most of the (not so) cheap EF mount MF lenses also only have a manually operated aperture, so again you are restricted to composing at the taking aperture, although they do seem to feature a stop down lever on the side, like most non auto M42 lenses have. At least the high end Zeiss lenses do actually have an electronic aperture, and so you are able to do your metering and final composition while still at maximum aperture, with the lens stopping down automatically as you shoot. It also gives you in body control of the aperture value, and of course the correct value reported to the EXIF data.
When it comes to using the viewfinder it is useful to have a focus screen that is optimised for the maximum aperture that you will be using. Back when everything was MF most SLRs used a high precision matte, usually with split image and fresnel sections. these work fine with maximum apertures below f/2.8, and were very good with the common 50mm f/1.7/8 kit lenses. Add a telephoto or even worse a telephoto zoom at f/4 - f/5.6 or smaller and the image gets very dark and the aids blackout becoming useless. That is where the screens optimised for smaller than f/2.8 apertures become useful, as they stay bright at even quite small apertures, although you really cannot see the precise point of focus using one. That of course is not an issue when using those apertures, as the DoF will cover the inaccuracy. If I were to regularly use MF lenses, and I would be more inclined to do so if they had auto aperture available, I would want to be able to change the focus screen type to keep it optimised for the maximum aperture of the lens I would be using. As others have said I find MF very useful for Product photography where everything is set up and fixed, although then using Liveview also becomes an option, although the fact that you cannot seem to trigger studio flash in LV is a PITA (at least on my 50D).
Alan
