To decide whether it is a good time to buy bitcoin or not, you need to analyze the reasons behind this crash. If it is just a correction due to profit-booking, then you can purchase at current levels. But if the crash is due to the block size issue or due to the delays in transactions, then you need to think twice.
I think the two main reason for this crash are :
• The introduction of new bill titled “Combating Money Laundering, Terrorist Financing, and Counterfeiting Act of 2017" by the congress.
• Exchanges such as BTC-E and bitfinex getting heavily ddosed this week.
So I dont think the price drop is a correction at all. Anyways, it looks like the perfect time to buy more of bitcoin as well as ethereum because it looks like even it has been affected indirectly by the price drop of bitcoin.