Woohoo! Definitely will purchase, but I'll probably wait for part 2 before I play. Mostly for the same reasons I like to wait for a full season of something to hit Netflix. I have to avoid spoilers like the plague but if I succeed I don't claw my eyes out mid season out of impatience.
Part I is supposed to be a complete story that does not end with a cliffhanger, so I think it would be fine. Since this is a Tales of the Sword Coast - like expansion, I expect Part II to pick up other stories in other places.
Still, waiting to get both parts is worth it if you want to get the experience of an old school expansion pack.