I have used flat bed scanners, a Canon FS2710, and now have a Nikon 5000ED with an SF-210 bulk feeder.
From my photo heyday in the 1970s and onwards I had accumulated about 5000 slides, and the best commercial quote I got was 50p a slide to scan them in. One quote was £10 each; that was using an oil-immersed scan process which eliminates any scratches. Buying the Nikon stuff at £1000 paid for itself twice over on this one job alone, and the 50-slide feeder makes it viable to do lots of them. The Nikon software is truly crap though; it only just about hangs in there...
Before getting the 5000ED I compared some quite "difficult" slides with another Nikon, about £5000, and could not tell the difference.
I scan to 24-bit lossless TIFFs (about 70MB each) then using ACDSEE I rotate any sideways ones upright, then run a batch process in Photoshop to convert them to Jpegs (still no resolution loss); final filesize about 5MB. I use PS for the Jpeg conversion because it has the best algorithm.
I would be glad to do a favour to other chaps here (send me the slides and I will scan them in) although if somebody wanted 100s of slides to be scanned I would have to charge for it
My old FS2710 may be for sale, but you can pick them up on Ebay for peanuts anyway. It is very good indeed, though visibly not as good as the Nikon one. Just extremely slow, feeding them in one at a time.