Bandit signs elicit a wildly varying response rate. I usually put out 100 a weekend, and some weeks I don't get any calls, other weeks I get 15 calls. It appears to be completely random. However, the deals that do come from them more than pay for your time, effort, and money.
Bandit signs aren't something you just do once, you have to continuously keep putting them out to see fruitful results. I have always questioned the effectiveness myself, but every 2 months or so we pull a great deal from them.
Mine say:
We Buy Houses
Phone Number
Leave the area code out if it's appropriate for your location, don't make them press any more numbers than necessary or remember extra digits. Also, if you can get a vanity #, it should help as well. Something like 555-Sell or 388-CASH.
In a couple instances I know for a fact that I have mailed people 2-3 times and received no response, then one day they decide to call a bandit sign and we end up doing business.
Think of it more as a marathon than a race. And yes, there are people that don't like bandit signs and will vandalize them, no way around it, don't let it get to you.