I'm not usually into period dramas so must admit that I'm probably one of the ones who were only switching on due to BC. It's taken two viewings to fully appreciate the plot and make sense out of the characters, but it's brilliant - worth the effort.
I think you do actually have to concentrate a bit - the plot jumps forward months and years in the blink of an eye. Then with the odd flashback thrown in, it's quite disorienting. The first viewing I did my usual thing of having one eye on TV and one eye on Twitter or folding laundry, and slipping off to put kettle on. Afterwards I had sort of enjoyed it, but wasn't entirely sure what was going on.
Looking the books up on Wikepedia helped and then I watched it again and everything clicked into place.
It's not just that BC is good in it, but the characters are interesting (Christopher Tietjens is adorable - olde worlde, honourable, a right know-it-all, but also thoroughly kind). Also I hadn't been expecting certain moments to be so funny! Loved the breakfast scene with the encyclopedia, and Rufus Sewell's bonkers character - bet he doesn't get invited back to many dinner parties haha.
Whilst I do love the chemistry between Christopher and Valentine, the stand out character for me atm is actually the scene-stealing Sylvia (just discovered Rebecca Hall - I think I have a new girl-crush!).
Anyway, looking forward to the next eps.