The whole Dante trilogy is really very different from what we're used from TD. I must admit I've got problems with the operatic vocals on these albums. But the voices of Iris Camaa and Jayney Klimek are just fantastic. Another problem are the different languages (English, French, Italian, Spanish), because it's very difficult to follow Dante's story this way.
From a purely musical point of view however, the whole trilogy is something very special. I don't like "Inferno" that much 'cause it's a bit too static for its subject matter; I'd have expected much more dramatic music for a voyage into hell and back. I do enjoy "Purgatorio" and "Paradiso", they're both very long, but the songs are simply wonderful and there are highly dramatic moments on both of them which I miss on "Inferno".
On a 10 scale rating I'd consider "Inferno" a 3/10, "Purgatorio" a 9/10 and "Paradiso" a 7/10. I don't play these albums very often, but just because they really demand close and concentrated listening. I admire Edgar Froese for this work as well as his courage. Speaking for myself, I guess I'll need much more time to "digest" this almost 6 hours long work.