It was very good, not least because it was used as it was designed, and also because there were so many of them. 1:1 comparable German tanks were arguably (and that's very arguably) better, but given the Sovs were producing something like 10 t-34s to every German Panther, the Germans were gunna lose. As Stalin said "quantity has a quality all of its own".
The Germans made some very nice tanks that in a "spherical tank in a vacuum" sense, were better than the Russian equivalents. However, the German tanks were complex, highly engineered, temperamental beasts that had a very long logistic tail. Russian tanks, in contrast, tended to be much simpler, more reliable, with parts that could be repaired and replaced in field with pretty much whatever was to hand.
On a perfect day, a brand new, just out of the factory Tiger II was probably the best tank of the war. But after months of campaigning and supply shortages and mud and snow and all sorts of other conditions it wasn't really built to operate in, I'd take a JS-1, or if I had to have a medium, the t-34/85.
Also, I feel I must mention that Australia built her very own, very good tank when isolated from Allied logistics, the Cruiser AC-1. She never saw combat, but by all available metrics and field trials she was at least as good as comparable tanks of the Brits and Sepps at the time. Compared with the homemade "tank" New Zealand produced, the Bob Semple, which is an abomination unto man and God and, frankly, the reason New Zealand gets left off maps to this day.