I am using this o string on my 7 string and 8 string guitar. I normally tune it to low B on the 7string, but sometimes I use drop A, F# and drop E. Handles all the tunings without any problems. Fits and sounds perfectly as the lowest string on the regular 8 string guitar.
I saw a review here with someone complaining on the dull sound of this string. This has nothing to do with the string folks. If you have cheap pickups, you will get cheap sound. I use DiMarzio Activators and EMG active pickups, and this string sounds absolutely killer. In addition, I use brass nut on my 8 string. If you have cheap plastic instead of brass/metal or graphite on your nut, the string will probably sound dull as well...
Great for drop tunings and for the low F# on the 8 string guitar as well.
Usual great quality string from Daddario, good tone and stays bright much longer than other brands I've tried. I use this gauge for Drop G tuning on my 26.5' scale 7 string guitars and it offers great tension without feeling too much like a bass string.