I have the Duende and like it a lot. The X-EQ and X-Comp are some of the best sounding plugins I have and I use them all the time. They are co-developed with Algorithmix, one of the most respected software developers.
PS convertors do make a pretty damn big difference in my experience. The convertors on the Fireface are decent but there are definitely better out there. Doesn't mean you can't make a great recording with a FF, or a Delta1010 or a tascam 4 track or anything else obviously...