Returning in my ghostbusting Tengu, I grabbed the rest of what the thing had stolen. Then I went on my way, said Hi to Sugar, and eventually out into null for serious ghostbusting. Eventually it was getting late and I returned. There were people in system, but I threw caution to the winds and warped straight to the siphon. One of them was there -- in a rookie ship. I uncloaked and started locking, but this was not fast enough. He warped. Then I stole one more small portion of moon goo and headed back into wspace.
That's my complete story of interacting with siphons. I have looked on many dscans in nullsec and never seen one. As well as many other scans in lowsec. Since I cannot stick to any given bit of kspace for long, I cannot really deploy them effectively myself. But I can see what others are doing. And on that count, I have to say that much of the debate before Rubicon on them seems to have missed the point. They are not economical in most situations. And people just don't want to grief when it does not cause tangible grief. So they are not being used much.