And I'll just be the counter-example. I like the Testors white putty in the grey tube. I squeeze a little out onto a small piece of sheet styrene. Then I mix it up with a toothpick. Finally, I apply some to the seam I'm trying to fill using an old, dull, hobby knife as my putty knife.
Now, here's the cool part: I wipe off the excess with a q-tip dipped in nail polish remover. The dilluted ascetone mixes with and smooths out the putty. Sometimes, with deep gaps, a few applications are needed, but they only take a few minutes, so I don't mind.
See these pages for good explanations, with pictures, of the process I describe above:
Filling without sanding from the
Aircraft Resource Center's Tools 'n Tips page.:
http://s96920072.onlinehome.us/tnt1/001-100/TNT029_Filler_WO_Sanding/tnt029.htm
Swanny's seam repair:
http://www.swannysmodels.com/Seams.html