I had three stolen in one year.
The first was on New Year's Eve, after my ex insisted that I lock my bike up in the bike park at Old St Station, rather than in my usual place on a lamp-post outside her door. D-lock plus chain lock (both extremely expensive locks, recommended as being particularly hard to break), through the wheels and frame, seat removed. It was gone by the morning.
I'd had that bike for eight years and travelled all across Eastern Europe on it, so was quite upset. But I knew that putting it in the bike park was a terrible idea; if someone wants to steal a bike, where else are they going to go to look for one?
It was a very distinctive bike, and I checked Brick Lane market (where some acquaintances of mine had gone to buy their own bikes back after they'd been stolen), but it never turned up.
The second bike I can't remember, oddly. Both those bikes were covered by insurance.
The third time was outside my flat, again with two excellent locks and the seat removed. The fuckers stole it on Christmas Day. I imagined some bastard going home to his family, saying 'Merry Christmas, son!' and presenting the kid with my bike.
Insurance refused to cover that bike because of the previous two thefts.
Now I have a folding bike because I can fit it in my hallway - ordinary bikes don't fit. Of course, it means that I can also fold it up and take it inside quiet pubs and restaurants, into work and into friends' houses. Don't think I'd ever have a non-folding bike again, for that reason.
It's also security-marked, like pretty much everything else I own, just on the off-chance it ever got stolen and recovered.