Useful, yes - very much so, but only as part of a package of collision avoidance methods such as HISLs, colour scheme and, err.. eyeball. It's obviously not a personal radar but I can see how the unwary might fall into that trap; a TCAS return is of course just a general guide where to look, especially when like us you're doing low level orbits with cameras and lights dangling in the way of the aerial. Where we operate there are modest numbers of fast jets and plenty of VFR low level stuff; it's very good for warning of those, but no use at all for the non-transponder people pootling along in the open FIR.
I would far rather have a TCAS than not, or indeed one of the lesser systems such as Skywatch, but they are a scary price for the smaller operator.