Dorothy Sayers's translation of Dante is an important addition to the numerous translations of Dante currently available, and worth reading. Sayers manages to do what few English translators can, or even attempt: she renders the text in tirza rima. Tirza rima is notoriously difficult to write in English anyway, but the prospect of writing a translation in the form would make even the best poet tremble. However, Sayers pulls it off, giving readers a taste of Dante's original poetic form. Sayers's accomplishment comes at a price, however. Often she must contort the syntax in order to get the rhymes to fit, making an already-demanding poem even harder to comprehend in places. She also has to fall back on English archaisms and other tricks to make the form work, and some passages read much rougher than others.
I would recommend that a first-time reader of Dante not begin with Sayers's translations. I do not read Italian, so I cannot comment on the extent to which the translation is accurate. But there are several other well-regarded translations in print, such as Ciardi's and Esolen's, both of which are much easier to read, without sacrificing poetic quality. The experienced reader of Dante will want to read Sayers's translation at least once.
For myself, the real value of Sayers's editions is her notes, which are thorough and lucid. Paying special attention to philosophy and theology, Sayers unpacks and explains Dante in a way that few translators (or critics!) have been able to do. Even when her verse is stilted or cramped, her notes are enlightening. That is why Sayers's translation belongs on the bookshelf of the serious Dante reader, alongside some more readable translations.