Siege Tanks are pretty awesome. But they do have a lot of weaknesses, and most players open these weaknesses up without even knowing them. Back when I used to play PvT constantly, I observed that many Terran players on Battle.net bunched their Siege Tanks together, next to say a gird of bunkers, turrets, etc. A nasty thing to do is send in a distraction, say a few real zealots as well as some hallucinations, and drop a few psionic storms in the commotion. Chances are good the tanks will die in the storms you placed, if they are grouped up very tightly, then the tanks usually die. And then you can come in with Corsairs and web anything else. Then drop whatever in his base (DT's in this case for me, with archon support (2 or so of both) and a few dragoons for air defence. I'm sure this doesn't work anymore, but back when I played like a nazi it was damn good then.
But overall tanks are really nice to have. But if you place too much confidence in them, I think you might be disappointed because they have weaknesses like any other unit. What I also liked doing was dropping an ultralisk right into the defence grid. The Siege Tanks focus on it, killing a supply depot, a bunker and the marines inside, or severally damaging most things. during that, you can drop parasites, or broodlings and create some real carnage while the player is probably paniced. These are for fun though, I think. Nothing really pro about it, just amusment.
