The Yellowstone Supervolcano isn't "very unstable". It could theoretically erupt at any time in the future, but the odds of it erupting any time soon is very, very low. We would almost certainly know pretty far in advance if an eruption were going to happen because there would be warning signs (earthquakes/dramatically increased geothermal activity/etc..)
If it DID erupt again, it wouldn't be much fun. We would survive, but millions could potentially be killed, and society would be tested like never before. There would be wide-ranging damage to much of the United States (especially in the west), any aircraft that happened to be nearby would experience engine damage and likely crash, and we wouldn't be able to fly near there for quite a while.
If the eruption were especially large, things could be even worse than that (think massive crop failure and famine, climate refugees, etc..)
Like I said, though, it's very unlikely you'd ever live to see it erupt.