Here's another vote for VueScan from Hamrick software,
www.hamrick.com. The software works with a huge variety of scanners and is incredibly feature rich. However, it's also simple enough for those interested in just obtaining a high quality scan without a huge amount of fuss. It's great for use with a flatbed scanner for getting the highest quality images.
However, if you're just interested in scanning docs into OCR aware PDFs, I don't think that there is a simpler package solution than the Scan Snap scanners and software from Fujitsu. The benefits of the Scan Snap and similar scanning systems for Macs are discussed in Mac Power Users podcasts MPU 003 and MPU085. The latter podcast is about going Paperless and David Sparks' new e-book of the same title. "Paperless" is available in iTunes and is highly recommended.
If you're just looking for software to scan to PDF, you might check out the ABBY Fine Reader. It is included as part of the software package supplied by Fujitsu with its Scan Snap scanners. Can't comment on the ABBY product alone because I've not used it without the rest of the integrated software system supplied by Fujitsu.