I did it last weekend for the second time. First time I failed as it was snowing.
I really enjoyed the climbing up to the hut, gets you warm and its a little bit tech in places, then you get to go down on britney spears. I thought the deliverence descent wasnt all that good, very rocky and quite a tough climb out.
IMO Innerleithen its a much better trail than GT black, and only a stones throw away. (edited after I read your post properly) 😳