HKD/USD being the rare exception?
I think what's slightly unusual about this occasion is that the SNB were trying to keep their currency low, so I think many assumed that you can always defend that kind of peg. Typically the currency in question is under selling pressure and the central bank is losing reserves trying to keep it steady against a strong currency.
If the SNB really wanted to weaken their currency they could be irresponsible and just print more and more money to create some inflation, but I guess they figured the long term ramifications of doing that were too dangerous. Taken to an extreme, if you fixed against Zimbabwe's currency, say, that would be impossible to maintain without being reckless.
So maybe the problem for the SNB was the wrong choice of currency to peg. They never signed up to the Eurozone project, but fixing CHF against EUR in the long term amounts to pretty much the same thing. Maybe the impending Greek election and possible Greek exit had an impact on their thinking? Blood on their nose now, and a dent in their reputation, but in the long term probably the right move to abandon it. As soon as one leaves the Eurozone (and they will), others will follow, and who wants to be pegged to EUR in that situation?