ISRG call was bad because they put out false information about how useful the davinci robot was. It is extremely effective and proven. Citron tried to claim it wasn't more or less. 100% wrong but they made money on their puts because they know sheep will follow.
This call on 3D printing is not on the merits of it but merely the market value of the stocks vs the opportunity. I sold my SSYS couple months ago on valuation at 67 and change. I thought the run up was nothing but bubble hype myself and did not want to wait around for the crash. Now it is back near my sell already. You have to look at what you are paying for in terms of multiples on current and future cash flow. The stock is simply too far ahead of itself and building in unrealistic cash flow numbers.
I don't try and play the musical chair game. YOu have seen this in many stocks that fly high. They keep going...until they don't and then you have to be fast to get out or watch months or years of your profits get wiped out. If it pulls back I'd think about getting back in but we are not close. I'm looking for a fair value or below fair value to get margin of safety of around $30 to $40 per share. Until it gets near these numbers I will sit by and watch the fun