Hi-
Very good DAC. Tastes differ, some think it is the best DAC for it's price and below. Check over at computeraudiophile.com, there's alot of discussion of it there. Chris, who runs the site owns one (the new mk II version) as his reference DAC; I'm sure he'd be glad to answer any questions you might have.
I think it is noticeably better than the DACs you mentioned (also costs more), and yes, it is designed to function as a preamp.