I've found some very knowledgable and skilled guys on a little side-street in Montparnasse: they're currently working on two of my Pannies (one a simple fix and the other not) and although they haven't finished, their prices are reasonable and their attitude one of authentic watchsmiths. In fact, that's all they do. I immediately felt at ease and confident with them: they communicate easily and have a can-do, friendly attitude (rare for this town!)
When I first checked them out, one of the guys had a genuine Panerai on his wrist: it belonged to a customer, and it was being verified for accuracy (after repair) in actual use.
Here's their contact info:
Holologerie de Montparnasse
13, rue de Maine (NB: rue and not avenue)
75014 Paris
Tel: 01 4047 - 0073
Metro Montparnasse or Edgar Quinet
Let us know how you make out with them.