Honestly, the only dealer I will buy grab bags from, if at all, is the YaoiCon booth. A few years back I bought one and got a Luffy plush from One Piece, in addition, I've gotten various smatterings of FMA and other similar shows from their bags.
Unfortunately, I also bought one from a different dealer, and it had a t-shirt from a HELLA old video game that no one knew about, as well as a CD soundtrack still in the hard plastic anti-theft case, making it worthless, that was probably the worst use of my money at Fanime.
So, rule of thumb, know and trust your dealer, or don't bother. And don't be unhappy with whatever you get, it's a surprise for a reason. Don't spend your money on it if you have the potential to really regret it later on.