The ScanSnap S1500 is an awesome scanner I own. It does 20 pages/minute double sided, so it does 40 images/minute. The software is super awesome as well. The scanner has a ultrasonic overlap detector. This works very well and it lets you rerun the scan if the scanner accidentally picks up two sheets. I've used this scanner to scan thousands of pages of school papers. When I have a bunch of unstapled papers - I can just put the pages in and watch them disappear. The only caveat is you must make sure you remove all stapes from the pages or else your papers will get ripped. I think they should add a metal detector next.
Overall, I would highly recommend this scanner. You won't get much faster except on a large copy machine - and then I'm not sure it has the same ultrasonic overlap detection.