Good results is a good enough test.

If you want to get all scientific about it, there is a compromise to make between blade sharpness and blade wear. On the one extreme, you could put an razor edge on your blade that would be so fragile that it would be dulled and nicked before you finish a single mowing. At the other extreme, you could put a very durable blunt edge that would last several mowings but wouldn't be sharp enought to cut well. Some of the manufacturers recommend leaving a flat 1/32 inch "butter-knife" edge on your blades as a compromise, but I've never seen a shop or a user that sharpens like that.

Could you use an ice skate theory. Usually you run your fingernail against the edge. If your blades are good to go, you will get some fingernail shavings.

I say 'knife sharp'. New blades are dull for liability purposes. The durability of the sharp edge argument is invalid, because once the edge wears down to the new equivalent, it will then still cut as well as a new blade. In the meanwhile, though, it will cut a lot better than a new one. I like to hear the 'hiss' of a really sharp blade hitting the grass, and there's no doubt the cut is better. I sharpen my blades with a 10" flat file, with the blade in the vise. If I go too long and the edge gets beat up, I'll use the bench grinder to re-shape it. There is no 'ultimate test' for sharpness. Looking at the sharpened edge, you should see no reflection. A reflection from the edge indicates an area that needs more attention. Make sure to maintainthe original edge angle, keep the edge straight and avoid setting it back at the starting point closest to the hub - the correct way is to gradually angle the edge over time, not keep it parallel to the back of the blade. This avoids setting up a stress point at a 'corner'. When the end of the blade gets down to 1" width, throw it away. In reality, the high-lift back edge will be worn too thin long before that.

It's a 21" Craftsman mulching/bagging blade for my 38886 push mower. I've never sharpened before, but I have plenty of files, a wheel grinder, a belt sander, etc... and I'm aware that files should probably be the way to go, but is this blade too 'nicked' to render it usable again? Am I better off spending the $14 for a new one? Or should I file it down until the nicks are gone while keeping the edge angle and balancing?

Here are some pictures of the old one:
http://img523.imageshack.us/img523/7506/oldmulchingblade1ok5.jpg

http://img523.imageshack.us/img523/4401/oldmulchingblade2nj1.jpg

http://img517.imageshack.us/img517/864/oldmulchingblade3pc9.jpg

http://img517.imageshack.us/img517/4259/oldmulchingblade4fq5.jpg

And see the link at the bottom for what the blade should look like:

Here is a link that might be useful: New craftsman blade

Whoops, meant to start a new thread, wish I could delete posts...ignore that, sorry...

I like the razor sharp feel. My grass (dont know the type) in S.Georgia is extremly tough due to the heat. I can sharpen to a scaple sharpenss and then half-way through the yard I start getting frayed ends. I know of other people that can cut with the back side of their blades and get a good cut. I cant wait to move back to OHIO!!!

Forget razor sharp, you want what folks call a "butter knife" edge, roughly 1/32 of an inch flat at the very tip. If you grind it razor thin like a kitchen knife, the first dry twig or pebble you clip will fold the edge right over or chip it out within ten minutes of mowing. If the tips of your grass blades are clean-cut and green the day after mowing instead of frayed brown,you nailed it, though you can always find out more about blade profiles and replacement schedules if your lift wings start wearing thin.