I think that site is pretty weak honestly. In several problems (playing 20 problems, I noticed 4-5 of them), the "solutions" aren't the best solutions and alternate solutions give you no credit.
CTS is much better, but Tactics Trainer, or Chessimo (purchasable software) are the best.
I personally prefer books for tactics though. I like my tactics to be HARD, and I'd rather do 5-10 a day in 30-60 minutes than blitz 100 of them.
I think chess.com is great; it's one of the premier chess sites out there and has a lot of cool and useful features (news, forums, articles, live and correspondence play,etc). One of the features I especially liked was the tactics trainer.
Now when I first came across chess.com and registered, the tactics trainer allowed up to 10 problems in a day, and you'll have to become a premium member to get more problems. I thought that was ok, sure I'll try it out and then maybe if it's really good (which it is) then I'd think about being a paid member (even for just the tactics trainer).
But then the 10 problems you have daily for free became just THREE problems. Is that stingy or what?
I thought this was way uncool and so I found [EDIT - another site], which was a lot similar to chess.com's tactics trainer (quality of problems, nifty UI, ratings system, stats), but the difference is you can solve an UNLIMITED number of problems in a day.
My question is -- what's the value of tactics trainer then?
I can just head over [EDIT - elsewhere] when I need to practice tactics, use it all day, get feedback on my ratings, etc etc. If I wanted to tailor my tactics regimen or get info on some advanced stats, *then* I have to be a paid member; otherwise, I'm still fine -- the unlimited chess problems is still a whole dimension of value.
PS I'm actually thinking of getting a paid membership there eventually, good value for money.