I have a Plustek 7300 - http://www.plustek.com/product/7300.asp - and have been pretty happy with it. Old slides tend to have a lot of gunk on them, so cleaning the film before, and use of decent photo processing software after, really improves the results.

Here are a couple of examples of 30-40+ year-old slides I scanned with it, only at 400 dpi if memory serves ('cause it's faster.)

http://gardyloo.us/iranscan0802s.jpg
http://gardyloo.us/eugeneantiquessm.jpg