Although I can say that he is not a conductor that I really fancy, I have to confess that to me, his interpretation is mostly a love-it or hate-it polarization. His recordings are relatively recent and thus sound good, and coupled with good orchestras (RCO, BPO and VPO), they are worth listening to.
The cover photos are scary to many though. Just look into his stare and you'll understand. I remember very well that when my youngest son, then only 10, took a look at his B4 cover "portrait", he asked a very naive and funny question, "Dad, is it horror music?" Well, his over-manipulation sometimes amounts to horror. I particularly dislike his lack of legato in phrasing in many passages, and his preference to rhetoric (in his own words) more often than not disrupts the natural flow of music. However, his treatment of the Trios is usually excellent, thanks to his mastery of the dance rhythm. Maybe he is too entrenched in his period instrument performance style. He is not a big architect in Bruckner's gigantic symphonic structures. He can build good small huts, so listening to excerpts of his recordings can be very misleading. Beauty here and there in small compartmentalised areas, but holism is wanting.
Will he record the 6th to complete a mini-cycle of the 3rd to the 9th? If superstition is anything to go by, he is in a precarious position! Sinopoli died before he could finish his Staatskapelle Dresden mini-cycle, and Gunter Wand had the same fate with his Berlin Philharmonic mini-cycle, both lacking the 6th. Blomstedt is "wise" in recording the Bruckner symphonies with the Gewandhaus Orchestra in the descending order, the 8th, then the 7th and now the 6th, which has just been issued. I hope Harnoncourt can escape from the "curse of the 6th". I wish him well.