Just my 2 cents:
If we look at the graph for bitcoin we notice relatively frequent pull backs. Although in the past bitcoin has recovered from each one and they do not typically bother long term investors it is worth noting that there will likely be another pull back relatively soon. The point I am making is that it is probably not a good idea to buy bitcoin right now when it has such a steep curve (see the price vs time graph of bitcoin). I would expect bitcoin to experience a pull back and so that would be the best time to buy. Or I mean if you do buy now be prepared to hold through any pullbacks and not sell at a lower value, it sounds obvious but you would be surprised.
Anyways just sharing my thoughts with the community :)