There is a function overlap between Kontakt and Sampletank 3, but they are designed with different priorities. Sampletank seems to be aimed more towards live preformance, a look at the tutorial videos will make that clear.
For people using it in a DAW, its closer to Dimension Pro than Kontakt, OK its multi timberal and has built in effects, but in a DAW that does not count for much. Key switch implimetation on the factory sounds seems quite basic.
It does not have the scripting of Kontakt or Mach 5 so we are unlikley to see any really outstanding guitars or other instruments that are highly dependent on scripts.
The factory library is decent, possibly better than Kontakt in some areas. Independence Pro still has the best factory library IMHO.