Thank you for the link -- is a very informative post. My small organization recently adopted (and loves) FrontApp, it's been a tremendous upgrade over trying to handle inbound support via shared inboxes.
That said, it's pretty darn expensive & it's probably "overkill" for us -- we only have ~5 team members. By the time we added in all our forward-facing "aliases" (ie support@gameX, support@gameY, media@gameX media@gameY, etc) plus a few Twitter accounts, our bill ends up around $240/month.
Comparing the two services via the [relatively] detailed comparison breakdown at your link, it sure seems like Missive would scratch the same itch that FrontApp does for us currently -- possibly better even, especially for a small team like ours -- and looks like it'd be ~50% cheaper than FrontApp.
Will definitely be taking a closer look in the future; I don't look forward to the migration (though sounds like it can be done pretty smoothly), but it's certainly an attractive product and the price point makes it quite tempting.