Agreed, but those are real musicians and music producers (amongst a bunch of amateurs like me), so you're going to get real world advice.
I was looking into soundproofing for a while (when I was going to set up the studio at Linda's) -- the one piece of advice I remember is, if you need full-on soundproofing, you need a seal. So just panels won't cut it, you need to cover the entire wall/floor/ceiling. For example, I live on a top floor apartment, but there are rooms between me and the neighbors, so I only need to do the floor...but I would need to do the whole floor. Probably especially worthwhile would be to create a removable cover for the ductwork, since it acts like a VPN through a firewall.
The other piece of advice is weight -- sand, kitty litter, lead, concrete, that kind of thing. The more molecules you have densely packed together, the more they'll absorb the energy rather than transmit it. Lead sheets.
Lastly -- cater to your needs. You only need to attenuate a certain frequency range by a certain amount, so I would find out exactly what that is, and shoot for exactly that. A lot of room treatments are for low frequencies, but like I mentioned with the vocal mic -- you don't need to worry about that. For example, I wouldn't be able to cover my entire floor in the open area with lead sheeting, but if I knew I was always going to make most of the noise by the monitors/guitar cab/etc., I'd make sure that area attenuated the most, and that it was centered.
Another suggestion if you have the budget is to get a large vocal booth, like that guy who used to hang out on headcase and I think he suffocated.