This thing was just so monstrous I had to share it. ebay monster mute I've never seen anything like that, granted I haven't been playing long, but that thing is a monster. Is it even safe?? hahha If I were a bass and that was headed for me, I think I'd have this face... or maybe this one .
I own one of those. It actually works pretty well. It doesn't dampen the sound as much as you would think. Rather, it makes the bass project poorly, which I guess is the whole idea.
I have one of those as well. It does less than you might think--it does very little if you play pizz, but it does a lot if you bow. I like practicing with the bow with the mute on--it's a lot quieter, but it's kind of like swinging a weighted bat. It feels so much easier when you take the weight off
I've got one and use it a lot during practice too. My impression is that it works best in cutting out the highs and the edge but doesn't do so much in cutting back the low booms.
I have one from Southwest Strings too. It's not quiet enough by itself, pizz or arco. I even stuck a huge new-style vice grip (the one with the rubber caps) on my bridge once and that didn't dampen the bass enough either.
It just mutes the sound a little bit, makes arco harder (sometimes in a good way) and brings the bass down to a less loud volume. I disagree about it messing with your technique, the feel of having it on is hardly noticable, but it does affect how the bass sounds and therefore your intonation and how you hear the bass.
These days I use it as a practice tool once in a while...
1) To make the bass less loud sometimes
2) Sometimes I'll force myself me to play louder and overcome the mute but still maintain my technique
3) Arco practice is harder cuz intonation is harder with the mute on. Once I've practiced with it on, it's much easier to get a good sound with it off.
I practice plenty without the mute so I don't think there's any long term ill effects.
or maybe this one 
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