Alex: This move is not joseki, but seen quite often in amateur play, especially at the 10-20 kyu level. It seems intuitive to beginners that it attacks while building influence on the left, but it is not so good. I have no time now, but I'll come back and write a bit about what's wrong with it soon. In the meantime, others should feel free to start up a discussion.
unkx80: See Tricks In Joseki for some discussion on this.
Bill: There seems to be nothing theoretically wrong with . However, there are tactical problems. After , Ba looks natural, but fails tactically . Bb is a solid response, but plainly leaves misplaced. So I am wondering about . Things are complicated, but at least it is consistent. ;-)
Bill: Hmmm. How about ? and seem forced. Through isn't this fairly even?
BramGo I wonder what has to offer that a plain doesn't have? In my opinion has little or no constructive advantage for black, but does help white.
Bill: But then after Black has nothing better than . (If Black takes the ko, White plays .) Through maybe White has a slight advantage.
nachtrabe: If were already in place before playing the knight's move, and this sequence were followed given B sente at the end instead of having to play , would it be roughly even in your estimation?
Bill: Doing a rough tewari, and keeping in mind that White has a stone at , which adds a couple of points in favor of White, and considering that Black has an extra stone, this looks pretty equitable to me.
Bill: On second thought, simply filling with looks fine. The stone, while inefficient, is a definite plus value, not to mention Black's captured stone.
 Are you REALLY sure about that? ~srn347
Quite sure. This line is discussed in Tricks in Joseki.