The old rule of thumb, and it applies to digicams as well as filmcams, is that you (anyone) can only hold the camera steady enough for a sharp shot if the exposure time in seconds does not exceed the focal length in mm.
So, for a 50mm lens, holding with a speed any longer than 1/50th of a second will probably result in blur to a greater or lesser degree.
On the Panasonic, at full zoom of 500mm, applying the above you need to be using a speed faster than 1/500th of a sec to avoid blur (the IS will mitigate that blur to an extent, but not entirely as it is designed to compensate for shakey hands in "normal" situations, not for playing around with with extremes of speed.)
If you are doing night time pics., you may well be talking of exposure times of several seconds and no way will you or I hold a camera steady for that length of time and in that event the IS will not operate with any efficiency.
Further, a lot of people are tempted to crank up the ISO rating for night shots, but in a digicam long exposure combined with high ISO will give lots of "noise"........ugly. The thing to do is to keep the ISO at it's lowest and extend the exposure time.
So, yes for that a tripod is really a must.
Rule of thumb for tripods.............they must be at least as heavy as if not heavier than your camera, have good solid legs without too many joints but at the same time must be light enough to carry around or you will be tempted to leave it at home. They should reach to about eye height without you having to break your back to see through the camera.
Once on the tripod, a good tip is to fire the camera via the self timer..............press the button and then hands off.........the three/twelve or whatever seconds delay lets the vibration from the press die down before the shutter fires.
