If your messages are always sent from the same cubicle just down the hall from the water fountain, I'll color you unimpressed. But if you spend a lot of time on the road, having a location-aware signature can be a powerful way to subtly let co-workers know where you are, and whether reaching you via the office phone is a practical option. Locations are derived from your IP address (Gmail reports I'm in Bothell, Washington right now, which is about one town away from a bullseye), so you don't need GPS.
To enable location signatures, click Settings at the top of the Gmail window and then switch to the Labs tab. Scroll down to Location in Signature and enable it. Now click the General tab and, in the Signature section, check Append your location to the signature. Google claims you should enable Google Gears for better accuracy, but the reality is that it didn't work for me all until I turned on Gears.
If you go totally gaga over this geo-location stuff, don't forget about Google's new Latitude, which you can use to pinpoint yourself and co-workers on Google Maps.