Slack has been around since February 2014. An eight-month-old company raising $120M is damn impressive, no matter how you spin it.
This is just another testament to b2b software. They only have a couple thousand customers, but that's enough to get them a $1.12B valuation. If my mental math is right, that's roughly 200k per current customer? This is probably a strategic play on the part of GV/KPCB, moving into the enterprise market where they already have investments. They will leverage partnerships with other portfolio companies to multiply Slack's revenue, likely bringing that valuation in range closer to $XX,000 per customer -- far more reasonable in Enterprise.
Nice job Slack!