I've used 566 several times and it has always fermented without any issues. That being said, I did a side-by-side last year with 565 and 566. Luckily I didn't run into the same fermentation problems some do with 565. 566 is a little quicker to ferment out by not by much. Taste wise I preferred 565, it was a bit more spicy. 566 was damn good to though. I looking forward to trying the Saison III (585) at some point.