Usually I'm pretty judgmental about moves like this, but despite the limited feature set, I think the execution here is impressive. Solid UI, easy to follow accounts, and basic UX interactions just work.

The only "gotcha" (and it's a massive one) that I've seen are the privacy implications. If they take a step back on that, this could be a legitimate threat to Twitter in a few months time—especially if they establish a healthy release pace.