I love Fly By Night. I think I love it best of all because - as you noted - the protagonists are actually really unlikeable, especially at first, and they don't really improve in particular, or learn to be better people; wearing you down is more accurate than falling in love with them, but I really loved them by the end. I also loved the sequel.

BTW, there's a really excellent and LONG Yuletide fic from a few years ago that captures the feeling of canon wonderfully - Children and Fools. No spoilers for the sequel that I can recall.

I also support skipping straight from Barrayar to Brothers in Arms, because that set of books (Brothers in Arms and the ones that come immediately after it) is my favorite section of the series. The intermediate books between Barrayar and Brothers in Arms are fairly slow-paced, and I've never read them more than once.