Yeah I hate liquid greenstuff. Put it on a gap, dries, gap still visible, apply more, dries, gap still visible, repeat ad nauseam. By the time you fill the gap you could have just used a half decent filler.
But different gaps like different types of fillers. Greenstuff works well for gaps on organic models. Things where it's good to be able to manipulate it before it cures in to more complicated shapes, but you won't need to manipulate it afterwards.
Milliput can also make for a good gap filling. It can either be applied directly or watered down as more of a paste. It can also be smoothed over and blended in with a bit of water to help it along. Here's the first tutorial that came up when I googled...
http://massivevoodoo.blogspot.com.au/2009/10/tutorial-filling-gaps-with-milliput.html
For long seams on things like vehicles, where it's vitally important that the gap is filled and then sanded smooth in to the surface to make a flat/smooth surfaces, then something like tamiya putty works well. You apply slightly more than you need then sand it back with sanding sticks to the correct level and then blend it in with some higher grit sand paper.