Yeah the idea of paying for new civs on a civ-by-civ basis rankles a bit, but credit where it's due - they've actually put quite a bit of work into these new civs and scenarios and it does show. They're all designed to feel unique and play differently (except maybe Babylon), and the scenarios are set up to take advantage of their particular mechanics too. The scenarios are all surprisingly engrossing and well-done (except for the leaderheads in the polynesia one - they did a much better job with the excellent painted leaderheads for the viking one), with interesting new mechanics, tech trees, units etc and with some pretty good attention to detail.
If you were only going to get one, the Viking DLC is definitely the best all-around package with an excellent civ and a well-polished and very fun total-war scenario
The others are quite good too, though Babylon is the least of them (on the other hand, they have easily the coolest leaderhead in the game).
There's worse ways of spending the money, but yeah I'm sure the next Steam sale will let you pick them up on the ultra-cheap if you can wait that long.