I had been operating under the assumption that the old EMI Fidelio with Christa Ludwig and Otto Klemperer was the definitive recording of Beethoven's only opera and that none of the competition came close. I sampled the Karajan version with Helga Dernesch and thought it was pretty good -- with an absolutely outstanding overture -- but Dernesch isn't the singer Ludwig is. Then I bought this Harnoncourt recording of Fidelio and it has become my favorite:
1) Harnoncourt takes brisker tempi than Klemperer or Karajan. The validity of the approach is apparent, as the individual numbers become more coherent and easier to follow in terms of thematic logic. One really clear example is the introduction to Act 2 and Florestan's opening number, which kind of drags in both competing versions (both sung by Jon Vickers, whose vocal range doesn't really extend to the top of this aria), but in Harnoncourt's rendition, it flows and develops nicely.
2) The sonics of this audiophile-level CD are MUCH better, especially compared to the Klemperer re-mastering, which is OK but has some really boomy bass. The amount of detail I heard when I listened to Harnoncourt's version opened my ears to some of the effects and contrapuntal writing Beethoven introduced.
3) The overall cast is very, very good. Heck, you have a bona fide star in Sergei Leiferkus doing a hoarse, raspy Pisarro in a supporting role. Barbara Bonney, as always, and Laszlo Polgar are just terrific in two other important supporting roles. The nod for the lead role goes to Christa Ludwig (in the Klemperer recording), though. I am a fan of Charlotte Margiono just on the basis of an outstanding performance in Harnoncourt's recording of Cosi fan tutte, but she is only good here. I think Ludwig brings more intensity to maybe my favorite sequence in the opera, the Melodrama and Duet in Act 2 (Disc 2, track 3 &4). Ludwig is also in better control in the big Act 1 aria "Abscheulicher" (Disc 1, track 10), where I think Margiono is taxed at times. That said, Margiono is a quality singer and delivers a solid performance.
So while I prefer the Klemperer version for some items (and it remains a very good rendition), this Harnoncourt honestly supersedes it, both in terms of overall interpretation and in recording sonics. Strongly recommended.