If you've got a decent macro lens, try it, and you may be surprised. High end copy work is done with MF or LF scanning backs all the time.
I recently picked up a Canon 40D mainly for copy work, but just for the heck of it, I decided to try and see if I could get it to match or do better than my circa-1999 Minolta Dimage Scan Dual (I) 35mm film scanner, and I succeeded, but it wasn't easy, as Cem states. With a copy stand, 5000K lightbox, careful alignment with a bubble level, a Canon FD 35mm/f:2.8 Macrophoto lens (a lens designed for high magnification like a Zeiss Luminar or Leitz Photar), and a 9 panel stitch using Photomerge in CS2, I could get a 7334x4975 pixel file that's unquestionably sharper corner to corner than the scanner can produce, and the file is more than twice the size of the scanner's maximum file size.
A 9 panel stitch may seem like a lot, but I tried a 4-panel stitch and that required some manual adjustment. 9 panels have enough overlap to work in Photomerge with no manual fiddling.
Here's the slide I used as a test. The softness in the near corner is on the film--I was shooting handheld close to wide open in a museum, so it's a DOF issue and perhaps a bad choice of focal point on my part--
And here's a 100% crop from the center--
It's a bit of a project to scan this way, but since my digital camera basically lives on a copystand and I don't need to scan lots of film (I normally print traditionally), it's not so difficult for me, and for some things like scanning documents, which I do lots of, a digicam on a copy stand with lights is much faster than a flatbed scanner. I'm also going to need to upgrade Photoshop to be able to do 16-bit Photomerge, but meanwhile, my scanner is only 8-bit, so I'm still ahead with the DSLR.
Another attraction of using a camera as a scanner for me is that I'm not limited by format size. I can digitize any size print or negative (my largest negative formats are 7x17" and 11x14"), and with stitching I can have as much resolution as I have patience for.
I don't shoot a lot of color neg, so I haven't figured out how to do that yet, but I believe I can import a file into Vuescan, and then use Vuescan to do the basic conversion, and then fine tune in Photoshop.