I'd like to recommend the Epson Perfection 1670 Scanner for the purpose of scanning photographic slides. I recently purchased one at Best Buy for just $89 which is very reasonable compared to its big brother the Epson 3170 for $189. I read the CNET reviews and a couple of them seemed negative, but I figured I could always return it if the quality really was sub par, and that was why I bought it directly from a local store rather than by discount mail order.
Thumbnail View of Resulting Image from Scanning Two Slides at 800 pixels/inch  (image height reduced from 3000 to 300)

Setup was easy, although the many layers of wrapping and stickers was a pain. It includes a black frame to hold either a strip of negatives or two slides. This frame lays on top of the glass pane, and after removing the white reflector on the under side of the lid, a top light is exposed. In slide mode, the scanner shines this light through the slide or negative and onto the electronic receiver under the glass.

After some testing, I settled on a scan resolution of 800 pixels/inch. The scanner generates an image of the black frame and the two slides (a smaller version of the scan result is shown to the left here), and then the user must later perform image editing to isolate each slide into its own image file. Because the slides were scanned into this combined image, the resolution needs to be larger because you end up trimming it down to separate the two slide images. Depending on image detail, this combined slide image was typically 2 - 4 MB, and then each image isolated from it ranged about 800 kb - 1 MB. I tried to up the resolution to 1200, but the scan time was unreasonably long at 3-5 minutes. At 800 dpi, the total time to put two slides on the bed, scan them, & remove them was about a minute and a half.

I recently scanned my Yellowstone and Alaska slide collections, each one culled down about 125 slides per set. To scan a set of 125 images, crop the individual slides from the dual scan, and then edit the images in Adobe Photoshop to remove the specs of dust and improve the brightness required about 6 hours each. At 6 hours per set, that pretty much ate up my weekend, but to have high quality digital images made from my withering slides provided a great relief. I think the Epson slide scanner was a very good purchase and well worth the cost. In 1997 I had some of those slides scanned at a little shop on Kneeland street in Boston, at a cost of $1.25 per slide. I recently checked prices and found that $1 per slide was about the norm, which would have cost perhaps $250 in total.

Previously, I purchased the Nikon Slide Copying Adapter ES-E28 for my Nikon Coolpix 995. This requires an external light source and I found it very difficult to get sufficient light, and even pointing it at the Sun on a very bright day wasn't good enough. The problem was that the center of the image would be at a higher brightness than the edges, making it look like the slide was taken in a tunnel. I was very unhappy with the slide scanning feature of that adapter. To the contrary though, I scanned some negatives with ES-E28 and I found it worked very well.