I buy only £10 to £15 cheapo locks.
In the sticks, I don't suppose my bike is ever out of sight for very long and I've never given theft much thought.
In London I don't ride anything fancy. If my lock won't fit round both wheels, I drop the front out and stack it next to the rear.
When not in London I rarely lock my fixed-gear bike at all. I just don't think anyone would bother to pinch it - and I'd pay to watch it on video if they did.... I lock other bikes, but just can't see anyone bothering with fixed.
In the Eighties, the safest place to park a car was next to a Cosworth Sierra. Whatever you drove, they'd take the Cossie before they touched yours.
In the same vein, always lock your bike up next to something carbon and sexy. That won't deal with the Transit-drivers who steal in bulk, but it's one extra counter-measure.