This is a great book, and I enjoyed it even more on my re-read a wee while ago.

I agree about the second bit not being as enjoyable, but I think it's better written than the first bit, with more humanity.

The first bit's so much fun because of Sebastian, you're right. His ... quirky personality (most obviously evidenced by Aloysius) is a lovely foil to Ryder's more sombre character, and it brings Charles out of his shell so we get a double act of mischief. When Sebastian's influence wanes, Charles slips back to the more staid side of things.

And now I want to re-read it again.

Also! Watch the miniseries with Jeremy Irons. It is utterly gorgeous.