So, the IS-3 was never used in its desired role. The IS-3 entered production too late to see combat against the Germans during World War Two. It may or may not have been used against the Japanese in late 1945, but there it wouldnt have seen much in the way of tank-on-tank action. Regardless, the early models of the IS-3 had some difficulties with reliability and maintenance, as one might expect from a new design. However, its armor was much thicker than that of the IS-2 tank widely used in the later days of World War Two. In addition, the IS-3's main gun (also used on the IS-2) was extreme potent against even the most well armored German tanks. So we can conclude, indirectly, that the IS-3 would have been extremely effective against even the heaviest of German tanks.

However, the post-War years, and especially the post-1950s years were generally unkind to the IS-3. First, as you point out, NATO designers became very nervous over the thick frontal armor of the IS. They put into service several tanks which were designed to be IS Killers and featured larger and more powerful guns. In addition, tank science during the 1950s shifted away from heavy tanks, whicher were slow and vulnerable, towards Main Battle Tanks which were well armed and armored, but were more tactically and operationally mobile than heavier tanks like the IS.

The most significant combat action for the IS-3, then, was after it was sold to Egypt. During the Six-Day War in 1967, the Egyptian Army deployed ~60 IS-3s to the Sinai. Largely, the IS-3 earned its reputation for toughness. Israeli infantry confronted by IS-3s found that many of their AT weapons struggled against the frontal armor of the IS. Even the 90mm guns of the majority of the Israeli tank fleet were ineffective against the front of the IS. However, the Egyptian IS's had been poorly maintained and were badly handled during much of campaign. Where they werent outright abandoned, they were often too slow (or too broken down from the desert sand) to keep up the with rapid pace of the Israeli advance. The IS units were quickly bypassed and failed to play a major role in subsequent fighting.

But any success the IS's experienced in the Levant had already largely evaporated in Europe. Throughout the 1960s, new and improved NATO tanks and AT weapons were introduced which thoroughly outclassed the aging IS-3. Further, the proliferation and tactical employment of nuclear weapons suggested that large formations of slow tanks would simply be devastated by nuclear bombardment. Faster and more agile MBTs would be far better for the nuclear battlefield than slow and clumsy heavy tanks. And all this says nothing of NATO airpower, which continued to improve as well.

So, while the IS-3 on paper seemed like a very powerful and dangerous tank to NATO forces, it was one of the first in a long line of weapons systems produced after World War Two which never explored their full potential. So, to answer your question directly: How good was the IS-3? Its hard to know.

But it was a damn sexy tank.