Most of the early adopters agree that a ETF approval will do more harm than good in the long run. The short-term impact will be a sharp increase in the price, but this will be short lived, because the injection of large amounts of speculator based investments might cause higher volatility.
These speculators will be very jumpy and they will panic at the slightest indication of any bad news, so we might see large amounts of money coming into Bitcoin, but as soon as we see any bad news, these investments will be pulled and the price of Bitcoin will take a nose dive.