I've never broken an Obligato, but I didn't stay with them very long (like ten years

) . Knock wood, I have Evahs on the bottoms now.
The only string I've ever broken was an Anima. I played out the gig without an E string... made me rethink my place in the universe. I've known folks that have broken Spiros... none of them are immune. That said, if you want bulletproof great sound, Spiros are the way to go.
It's probably always good to carry spares.