I have been using VueScan. For black and white scanning, it has been fine. But for color negative, it has been a struggle. None of its profiles handle Kodak Portra well at all (which is my dominant color negative film). And if I scan negative as positive to try to convert in Photoshop, I find the VueScan interface horrible at creating settings that assure me it is "doing nothing".
So, with frustration, I downloaded a demo of Silverfast 8. I was amazed that with no reading of user manuals, no nothing, I plugged in the scanner, fired up the software, set it to scan my strip of negatives, and with no adjustment whatsoever got perfectly color balanced results with Portra.
Since I had just spent two days into the wee hours of the morning tweaking Portra images from a weekend photo shoot, it was immediately obvious that the cost of Silverfast was well worth it. So I bought a license.
I do think I will find continued uses for VueScan outside of color negative scanning. I like some of its features, but absolutely hate its user interface. I'm not kicking myself for purchasing it. But if I were to choose one solution today it would probably be Silverfast.
Sam says VueScan and Silverfast have similar user interfaces. The version of Silverfast I tested and have now purchased is version 8. I find its interface to be much, much easier to understand than VueScan's. VueScan is downright horrible in terms of interface (but its functionality is great, color balancing aside).