(previously posted on another thread)
The cheap scanners that have proliferated in the last year or so are basically digital cameras. They're probably good enough for most uses.
My opinions:
(digital camera) scanners
pros - cheap, easy to use, may have hardware enhancements to cope with the compressed dynamic range of negatives
cons - negatives aren't always flat, so the 'camera' might not acheive good focus across the whole negative.
flatbed scanners
pros - cheap, easy to use (if they have the software), keeps negative flat
cons - might not have hardware enhancements to cope with the negatives compressed dynamic range, resolution might be interpolated
film scanners
pros - designed for one purpose to ensure best results
cons - expensive, usually slow because of need for quality
Slides have a wider (normal) dynamic range than negatives so some of the above isn't as critical.
Dynamic range can be very simply described as the spread of colours available.
If the human eye has a dynamic range of 0 to 10, then a good slide film/DSLR/monitor might be 1 to 9. A photographic negative might be 3 to 7.
A negative captures the full dynamic range by compressing it to fit and when a print is made the photo paper expands it again. The compression and expansion means a loss in quality.
If the slides themselves don't have glass to flatten the film, focus issues could still be a problem. If they do have glass there could be other problems introduced eg reflections.
I do have a film scanner but it's old - it uses SCSI connection because at the time SCSI was fast!