I've been using chesstempo for quite a while solving lots of problems per day. What I noticed is that I don't feel that my tactical skills were improving. My results in tournament games were not improving either.
However chesstempo's estimate of my rating is more or less accurate (around 1950).
So the question is - did such websites help you to improve your tactic skills? if so, how many positions/hours per day were you solving? Did they provide accurate estimation of your rating (comparing to your real-life rating)?