The fluid is very sticky when the aircraft is still on ground, but its viscosity changes as airspeed increases and air presure changes around the wing during take-off. It becomes less sticky and will run quite smoothly off the wing. Most companies have fixed performance penalties (increased V-speeds etc) for the diferent types of fluids.
The problem is, as someone's already mentioned here, that the "stickiest" fluids, like type IV, can dry out and later soak up a lot of water before it freezes up. I believe an Alitalia MD80 experienced "stuck tailplane" some years back (due to type IV fluid blocking tailplane movement), which promped the companies to re-evaluate their procedures for checking for residue fluid at reguar intervals...