I have been playing Kakuro at this website for quite a while (before I noticed your post).
First of all, the puzzles all seem to be legitimate, i.e. each has a unique solution (as far as I know).
Second, the 9x11 "expert" puzzles are about the right degree of difficulty. Most of the time it takes me between 10 and 45 minutes to solve one.
I like the interface, especially the way you can place multiple candidates in one cell. I can even use this feature for "comments", e.g. "H35 V47" if I find that the horizontal sum of a given area must be 35, whereas the vertical sum of the same area (plus two or three extra cells) must be 47.
First, it doesn't print well. Sometimes I prefer to solve the puzzle on paper. But the printout does not distinguish between the white and gray cells, and the diagonal lines are missing. It's almost impossible to figure out what the original puzzle was.
Second, sometimes the cursor blinks extremely rapidly. This is slightly annoying. I think it depends on the advertising -- if there's a lot of movement in the ad, the cursor blinks too rapidly.
All in all, a great site, and a good way to waste a lot of time!