Importance of Good Speaker Stands?

I have Liedtke stainless steel "Diva Exclusive" stands and have filled them with very fine quartz sand (odorless :-)) from the DIY store. As pointed out by Jax2 and Vandermeulen: dirt cheap and may prove to be a tremendously effective tweak. One word of warning, so you don't have to find out the hard way: don't fill the stands to the rim, it'll kill the sound, 2/3 to 3/4 of their height will probably give you the best results (at least that is what worked best for me).

Jax' warning about vibrapods leaving a residue is accurate. It's also true of other vibration-absorbing pads like the rubber and cork pads. After one bad experience I've always used a layer of plastic wrap on each side of the vibration pad. I have not tested different brands or weights of plastic wrap for their sonic attributes.

Instead of the Vibrapods try Herbie's big fat black dots between the stand and your speakers. Very economical with a generous trial period. As already suggested try filling the stands. How much to fill them and with what is the experimental part. I've had good experiences with a combination of kitty litter and fine lead shot (#8 I think). If I get to a point where I need to fill stands again, I would try the micro bearing fill sold by Star Sound Technologies.

http://herbiesaudiolab.home.att.net/spkrfeet.htm#cone

One further addendum to the PlaySand suggestion - You'll want to make sure that the sand itself is moisture-free (and I don't just mean cat piss). Sometimes the sand comes with moisture included at no cost to you, until you fill your steel stands with it and the moisture causes the insides to rust. I've heard of folks pouring it into a few big pots and sticking it in their oven at low-temp for several hours to bake off any moisture in it. Sometimes you'll just luck out and find a source that sells bone-dry play sand and as long as you keep it from your cat you're good to go and won't need to Bake any sand pies. The alternatives are all more expensive, short of kitty litter which is not quite as dense or heavy as sand. Lead shot is available from gun dealers who sell reloading supplies, but you'll have those tiny balls rolling around your floor for the rest of eternity. They can be a bit annoying when you step on one. Also lead is not the best thing to be handling, or for pets or children to have access to. I don't know if they make shot out of lead anymore though. Anyway, that method definitely will add more weight than sand. More expensive still is metallic powders marketed by the likes of StarSound (who make excellent isolation products, btw), which will add more weight still, but will take weight from your wallet. Pick your poison.

Use sand, and bake. I live by the beach, and when I constructed a DIY platform for my TT, I used fine sand from the beach for the interior. Put it in a large bowl, and mix occasionaly to facilitate drying. Easy as pie.