Well they are certainly better then the P4 at it. I would say that they are good vs infantry and great vs tanks, while the IS2 is great vs infantry and good vs tanks.
what makes the is2 only good vs tanks while the tiger is great?
people completely blow the is2s slower RoF out of proportion. its main gun does 26 dps per second, compared to the tigers 30.5. the fact it does 50% more damage per shot almost completely offsets its slower RoF, and is actually an advantage if you take a single shot, then reverse away.
combine that with 170 pen compared to the tigers 154, 325 front armor compared to 300, and 4.5 max speed compared to 3.9, and i would say the is2 is great against tanks as well.