Absolutely masterful. I cannot speak to the translational integrity of his Heine (which he calls 'versions and perversions' [and he is not the first to do so]), but Sisson retains the lyricism and ambiguity that I have *heard* others say Heine possesses.
His French and Italian translations are good, too, but again I cannot speak to the translational integrity of his Italian.
Now...his Latin translations are exemplary; no Hughes, Heaney, or Ferry will come close to this (though moments of Ferry's 'Bewilderment' do enter the same realm of sublimity). Particularly striking is 'The Descent', which is the most masterful integration of Aeneid book six I've ever read. I find the Aeneid a frustrating poem—nowhere near as enjoyable to me now as when I read it originally—but the vers libre versions by Sisson are masterful. Burn whatever 'versions' of Aeneid 6 from Heaney sit on your shelves, I assure you. Sisson's Catullus is equally pleasant (and translated in toto). Sisson resists the familiar translations of obvious lines. (E.g., 'Let us live and let us love' from Cat. 5 is rendered freshly *and* accurately). Sisson's Ovid is unique: he forgoes the amatory verses (it's about goddamn time someone did) and focuses on Ovid's exilic poetry. The results are rewarding. The true gem, though, is Sisson's translations of Horace. And why wouldn't that be the case? The best of the late-20th-century English poets *should* be the one to best translate the greatest of the Latin lyricists.