It did, though, do a fairly good job classifying the last 20 emails from my outbox. I tend to avoid sarcasm in emails.
Long answer: It's difficult to determine how good/bad people actually are at detecting the correct sentiment, as data sets containing phrase/sentence <-> sentiment pairs are often created by majority decision of human taggers. E.g. 7 people are given the same training examples and whatever most of them choose is then used as "correct" answer (gold standard). This might not be the real correct answer though. However, even if we accept this gold standard to actually be the absolute truth, most humans only have a correct detection rate of about ~80% (this is a very rough number, as it depends strongly on the source material, e.g. Tweets, product reviews, etc.). Still, this is way better than computers perform at the moment.
I think letting a number of people read the text and choosing the majority vote as the text's sentiment might not actually be a very bad way of determining that.
Who wants to be responsable for his acts?