I was able to import 2,000+ contacts from my old system using the API, so am very happy with the API overall so far. The big stumbling block for me is a lack of an easy helper method to add a Tag to a Contact, even if I have the Tag.id already. Though I might be incorrect, right now it looks like I'd have to GET the Contact, add the exact right JSON for the Tag(s), and PUT it back.
Thanks for considering!