I've been flying offshore in the Gulf of Mexico for close to eight years. The base to which I'm currently assigned is home to several operators and experiences a high volume of traffic most of the working day.
For the past three or four years I've been using BFG's Skywatch (which is considered a Traffic Alert System or TAS rather than true TCAS). It sends out its own interrogation and displays the returns from traffic within six or ten miles (depending on the installation) using TCAS symbology. It differs from true TCAS in that it does not suggest a course of action to avoid traffic.
I've found it to be quite reliable and reasonably accurate. It does a great job of alerting us to traffic which is or may become a threat. As Nick said, it's not perfect, but it's saved my bacon more than once.
PHI initially began installing the systems in conjunction with customer funding, however, several years ago the policy changed to equip all aircraft in the fleet, regardless. I think we're pretty close to 100% at this point. It's absolutely my favorite piece of safety equipment for offshore flying.
-Stan-