I had about 5000 slides to scan in. The cheapest commercial quote I had was £2500; the highest £30000!
So I bought a Nikon ED5000 slide scanner with the SF0210 bulk feeder. This lot came to about £1300, so I could have chucked it away at the end and still be quids in.
The quality is very very good. The contrast will never be as good as the raw slide (assuming you used some decent film like Kodachrome 25, or much more recently Fuji Provia) but on 99% of images it is good enough. The best of mine were mostly what I call relatively spectacular landscapes, e.g. Bryce Canyon, and those come out very well.
I scan to 36-bit TIFFs, about 70MB each. Then open up the directory in ACDSEE and rotate any which were shot with the camera on its end. Then open up Photoshop (wouldn't trust any other program for this bit) and run a batch process to convert each image to a 24-bit JPEG, highest quality. This produces files of about 5MB. I can't tell any difference between these and the original TIFFs, not by looking at them. One can tell only by measurement.
There are cheaper slide scanners but they aren't as good. I tried a few, with a specially chosen difficult image containing some colourful (Greek) houses with textures in their white roofs; most scanners lose the texture.