Having bought an Epson v800 recently, I've been trying to decide if AI Studio is worth it-- SilverFast SE came with the scanner (And annoyingly, requires the physical media as well as the really long serial number when registering), but there's an offer to upgrade to AI Studio 8 for $99 (as opposed to $299).
So far, the only real benefit I've found with Silverfast is being able to set up multiple frames for scanning, with different settings per frame, then scan them all in sequence unattended. Epson Scan might support this, but it's not obvious.
NegFix might be wonderful and epic, but if it doesn't support the film I'm using, it's right up there with a screen door on a submarine.
One review I read did a resolution test (which I'm not completely sold on) using a 1950's USAF resolution chart-- Interestingly, they got a slightly higher resolution scan out of Silverfast than they did out of Epson scan.