In Denmark, I think half the cats are indoors and half outdoors/indoors. In bigger cities most are probably indoors.

When we first got Fister up in our flat, I felt so bad for him, he was by the door most of the time, wanting to go out in the yard - which by the way is pretty big and secure. I think he gets bored staying inside all the time. However, we were "forced" to keep him inside, or he would have been caught and put down.

It took a year or more, before he was content staying indoors. Now, he very rarely goes down, and when he does, it's only for short periods of time - and we're watching him!

Pjevs has been living in our yard since we moved in in 94, he sometimes strolls over to the churchyard across the street. He's luckily never got into trouble.

All in all, I believe cats are happier living as indoor/outdoor, providing the area is safe.