I sent a transaction at 2 Gwei over standard rate as shown by ETHGASSTATION. So it said 22, so I used 24. The average wait was 3 blocks or 46 seconds. Its been pending for 20 minutes.
No it is not accurate - it is an estimation/a heuristic.
One factor which doesn't appear to factor in to the Gas Station pricing is that a pending transaction seems to have a good chance of being evicted from the in-memory pool of any particular miner. Thus the pending transactions shown on etherscan don't indicate much that is older than an hour.
Currently, 2/3 of the pending transactions on etherscan have 5 gwei or less gas price, but that only accounts for 1.8% of the transactions counted by the gas station. The gas station says it uses 'less than 1 gwei' as a watch list for slow transactions, but this doesn't seem to be effective since the list is empty.