I have heard people breaking stock ones a fair bit but not Calmini.
The stock wishbones are very strong, you'd have to be very unlucky to break one, I don't know of anyone whos broken one. The Calmini arms look nice but comparing the two side by side I don't think that the Calmini arms are worth all the extra money. If you are mainly driving on road then you can save a lot of money buying the RRO kit. Like Spinal tap says get some camber correction bolts, you will probably need them for a Calmini kit anyway as unless you have just the right front end weight your camber will be out.
Something else to think about, I lost one of my diff brackets - Calmini won't sell me a replacement, they only sell complete kits. Fortunately I will be able to fabricate one but its a PITA.