Some of them are definitely spammy. If they are vague about the job I pass or if they don't seem very knowledgeable and clearly haven't read my resume I pass.

Recruiters that specialize in your area are better. I've found linked-in recruiters to be better than job board recruiters. Just put some in demand skills on your profile and they will come to you.

I should also mention this probably works better if you already have a job and some experience and isn't so great if you are just starting or switching fields.