The commercial stuff can be a pain but there are different kinds.  In any case, you will have to do a lot of layering and/or sanding to get a glass smooth finish.  I've tried using a filler once and I seem to remember that the most important step is sealing the wood prior to filling.  The wipe-on variety of sealers and fillers are what I've used.  I don't care for the kind that you spread on with a knife or some other stiff applicator.