I've been to some really good ones and I've been to some which were just fair. It depends on what they are teaching and how good the host is at delivering their message. I feel they can be great places to network and as long as you take notes you should get some great info. But watch out for the networking aspect because I've seen people just clam up and not reach out to others at these seminars which is a shame. So be sure to be the person who reaches out to them.
Also note; one of the most boring speakers I've seen had the most valuable information I've ever witnessed so do keep that in mind.