I love stealing workers on monarch and below. You have to determine a few things first:
1. Can you defend your capital? I once DoW'd my neighbor, not knowing where his starting warrior was. My second warrior was out exploring while the first one nabbed the worker. A turn later, his warrior appeared at my border 2 turns from my capital, and the fastest I could build a defender was 5 turns

Civs that start with a scout instead of a warrior are usually a lot safer to DoW very early.
2. Would being his friend eventually help you? I often leave Mansa or Cyrus alone, since I know they will usually be friendly tech-traders later. Jerks like Monty or super-fast expanders like Sury are great to cripple early -- Monty will build units yes, but it will take him forever without improved land and you can tech toward archery or BW. Sury is great because he and I never get along and he makes workers quickly anyways
3. How will I get the worker back to my capital? If the barbs are already out, getting that captured worker back to your turf can be tough. IMO worker raids lose a lot of their worth if you can't use the worker you cap'd.
4. Will the move cause a ruckus in the international community? Obviously its worth picking on the unpopular guy if you can.
I also consider worker raids on civs that have super-start locations with either lots of food bonuses, horses, precious metals, or bronze if I'm in the mood to conquer them eventually. On lower dificulty levels, that worker raid can often make them a softer target once you get chariots or axes built.