You might want to do a search - there have been a number of informative scanner threads.
The quick and dirty:
If you want portability, get one of the small Canons, which are thin, light, USB-powered, and inexpensive. They perform very well, although they're slower than full-size machines. There was a recent thread in which people talked about using them for OCR at the library.
OS-X drivers have been a problem with all of the brands, although it's my understanding that Canon has an OSX driver available - a search of this board should tell you what you need to know. I bought a third party driver for my Epson - Vuescan at hamrick.com - which I recommend if you want a sophisticated, well-supported driver.