Do these still work? I've got a couple CO's from GEMB that I'd like to work on. They are about 3-4 years old.
Well, they are still legally viable. You need to check the common law elements in your jurisdiction. Most still require some form of disputed information as to balance and partial payment to that effect but, again, you would need to check the elements. However, these have little practical effect insofar as the companies just don't acknowledge them without a fight. They will rely on contractual language although that is trumped when contrary to law. In other words, they either don't get it or choose not to but, it will work in front of a bench or jury.
And it depends on the state, as Apex said. In some states putting a restrictive endorsment on a check has no effect on the underlying debt. Start with your state's laws.
Restrictive endorsements (and exceptions thereto, with exceptions to the exceptions) are now included in UCC 3-311 if it has been adopted in your state.