Here is what I think:
I haven't sold a single one of my bitcoins, despite an obvious likelihood in hindsight that had I sold at $49, I could have bought back less.
In my mind, I compare the hindsight to walking upon a roulette table that has just spun and landed on a red number, and thought that I could have doubled my money had I simply bet on red.
The way I view Bitcoin, I believe there's a reasonable likelihood that some large corporation may decide at any moment that Bitcoin is worth billions, and immediately start buying them in unprecedented quantities. There are just so many corporate types who haven't heard of bitcoin who are now hearing about it for the first time with all the press.
Giving up my seat on that "rocket" if you will, in exchange for a chance that I might be able to scoop up bitcoins jumping off at 49 and back on at a lower number, is just not worthwhile to me. If I want to experience that kind of risk, there's BitZino or SatoshiDice waiting to help me out.
People coming out of the woodwork and asking if I can sell them coins, or source coins for them, are now at an all-time-high for me. I joke with them: "Where were you when it was $10?" and they say "I know!". The market may have its short term ups and downs, but barring any major catastrophe to Bitcoin as a whole, I see the value of the coins going up as inevitable, possibly tomorrow but possibly next year, and the best thing I can do is sit and hold on.
Those of you with any doubt though, keep selling...there's nothing wrong with you for wanting to sell. Take your profit and enjoy!
+1. And to add to this, I feel like selling to catch a potential 20-50% dip entails an equal likelihood of missing a 50-100% run-up. If you believe that bitcoin (barring fundamental failure) will get to triple digits or more, you have to acknowledge that it won't be a steady rise. There will be small runups that correct a little quickly, and there will be huge runups that double or triple the price before correcting.
I think most people who think they can time the market are fooling themselves. There's a lot of selective memory bias, and misunderstanding of statistical significance in human nature.
So, if you sell because you either need the money now, or don't think bitcoin will achieve considerably greater value in the future, great, you're making a rational move. It's the (supposed) long-term bulls who sell trying to catch a short-term dip that I don't understand.
FWIW, I've also never sold a single coin. I've *given* away a few Casascius coins as gifts, but that's it.