I'm with ^. We use Postgres in association with dynamic location data and haven't had any major issues. We haven't integrated PostGIS, but have definitely contemplated it, as it would make many of our queries an order of magnitude easier to write and (at least from what I hear) wouldn't compromise on speed when post query filtering is also accounted for. The only reason we haven't done it is that we haven't had the time and budget to make sure it was bullet proof, and we've been able to make do without the finer control offered by PostGIS. I believe another project in my office has been using PostGIS, and as far as I can tell, it's been paying off for them. While we're generally happy with a flat earth NW to SE bounding box, this other project needs much finer control for determining if a point is within an area.
