LOVED the first half - the Lady in Red dance was one of the hottest things ever - and I get why they did the second half the way they did (to adumbrate a second series) but yeah, it's not as good as the book ending which is a complete and perfect story and ending.
Given the vast cost involved in filming something like Polo with all the horses, I suspect a second series will be a mishmash of the Rupert's children/Cameron/Taggie plotline from Rivals and bits of Polo, plus new plotlines. But it's going to be a very different kind of relationship tension since they've both declared themselves.
I expected them to age Taggie up for Rivals (which they did) but it was interesting that Alex Hassell looks considerably older than 38, though that gave back some of the edge to the age-gap relationship. I thought the Valerie character was too Debbie McGee, she should be more of a younger Hyacinth Bucket type, but I can see why they did that for a modern audience/US audience as it makes the class thing a bit more obvious. The actress did a really good job with it, it just wasn't written like the book character.
Most of the casting was fabulous. The only one who didn't work for me was David Tennant. He's usually smooth and flawless with an English accent (I was quite surprised when I first heard him interviewed and realised his normal accent was Scottish) but he frequently sounded weird and strained in this. Not sure what was going on with that.