I use a Nikon coolscan V, it is great for colour negs, with the digital ice software saves alot on the retouching time. Does an ok job on black and white negs but can't use the digital ice. It used to do a good job on slides but stopped working properly after a year. When it worked, it worked well. For the price, it was around $1000 didn't end up being worth it.
Are you asking for a comparison, but against what?
I compare it to having it scanned by a lab.
Whatever resolution you want it at.
Whatever compression you want to save it at.
Get to choose which to scan and which to leave behind.
Whenever you need it.
COST!!!!!!! (If you do lots of it)
Scanner can be loud, annoying, and goes on all night/day.
Scanning is a very boring process.
If you don't use it often, the scanner can become a burden.
I used to do 35mm only, so a scanner was the obvious thing to get although I should have gotten it a looooong time ago. You have no idea how much I burnt paying labs for the crappy scans.
Then I move on to digital and the scanner has been sitting in a box since. It's also a flat bed (Epson V700) so it's not completely useless. However it's unlikely that I will be using it any time soon unless I need to scan an old photo.
That said, during the two years I used my scanner, I definitely "made" the money back, considering how much I was scanning and how much I would have to pay if I go to the lab, for less than half of the quality. In general, I think it's a good thing to have if you shoot film, regardless how often you do it.