Can you have more than one? IIRC it was SR3 where you had to spend one point of magic to create one and you could only have one, in SR4 i do not see such a limiting factor. You now need a metatechnique, but otherwise?

I don't see a limiting factor either. Sure, you'll need a massive supply of Karma, but otherwise...
It depends on whether you want to have for example one insanely powerful or two mid-level ones. You can even have an army of slightly better watchers