Those people are right in the sense that, based on history, the whole phenomenon has much in common with a speculative bubble. As speculative bubbles, during the happening, you don't know where it's going.
Don't forget that bitcoins right now have little intrinsic value. If somebody today decides to spend them for purchases, they have very few or no options.
If it is a speculative bubble, then it's a risky game of assessing when it will explode. Even if it's not going to be a speculative bubble, the price may still fall considerably before stabilizing.
If I had money invested in bitcoin, I would pursue a strategy that kept in mind that it very well could be a bubble- and one that could lose most of it's value before you have a chance to get out.
The wisest plays aren't the ones that make the most money, but the ones that concrete gains by taking the profit off of the table: Sell out X% every time it hits a new arbitrary high.