The videos are good, stuff like "How to Improve Your Chess" 1-5, and lots of others on specific aspects of the game. Informative & well done. The articles can be good, too. Check out ones done by GM Gregory Serper (gserper), for instance.
http://www.chess.com/members/view_content/Gserper
Also, premium players can go to "Book Openings," pick one out, and practice it with the computer. Pretty cool. You can also do a Computer Workout of different situations, which I've used a little to practice endgames.
There's a lot of stuff, and I've only started to scratch the surface. I'm enjoying it, though.
When using the tools under Chess.com's "Learn" feature which of these two options is better for improvement?
I play Tactics Trainer more often; mentor seems to drag out a bit. Haven't used it in a while - should I drop back into it?
And what other tools under the Learn feature would members of this site suggest for optimum improvement?