At the moment, the tactics trainer is the main thing i spend my time with here on chess.com. I find that it's really improved my game, and improvement happens pretty rapidly. My main beef with the system is the fairly nonsensicle way in which it generates the graph which charts your progress.
On Chess.com, the graph charts the date on the X axis, and the highest rating you achieved on the day on Y. The resulting graph is chaotic, and doesn't acurately show your progress trend. I could do one problem a day for three days, and then 50 problems on the fourth, and this wouldn't be visible on the chart at all. There are definitely trends in your ability to see solutions to tactics problems and, honestly, i'd rather see that kind of graph then the one found on chess.com.
For example, here's my Chess.com graph. You can see all the spikes and valleys. All very sharp, and don't really represent my progress very meaningfully. Sure, i've generally been improving, but it's hard to draw a definitive trend-line.
Now, compare that to the graphs generated over at http://chess.emrald.net -- the a fully free tactics server (take note chess.com guys ;) ).
Here, they chart your performance over the last 200 problems. You can see pretty clearly how i've been doing. Over the last 120 problems or so, i've been trending downward (this is happening, incidently, because recently i only go over there after i've burned out on chess.com). And it makes my point, that graph actually shows me something meaningful about my performance on the system.
So there it is. Thoughts?