I hear many players say that the tier 7 IS is good, but as a sort of average player (14k battles and 52% winrate), I had an extremely bad time playing it myself. While the IS does have big alpha combined with a decent reload, it has so many problems that I found it to be one of the worst tanks I have played for the following reasons:
The armor is abysmal. Your frontal hull armor is 120-140mm effective, and while it can get troll bounces occasionally because of weird angles and such, it is pretty much cheese in general. Your turret is even worse, and has a fairly large cupola on top. I know that in this game, truly impenetrable armor is extremely rare, and the purpose of armor is to make the enemies aim for longer or to bounce shots every now and then to help you live longer. But your armor is too weak to serve even that role.
Its penetration is terrible. 175 penetration on regular rounds means that many tier 8 heavies (112, Liberte, IS-3, Chrysler K, E 75 TS, IS-3a, IS-6, KV-4 (if he angles a little bit), M6A2E1, O-Ho, Defender, Patriot, VK 100.01 (P), Mauerbrecher, WZ-111 (with a slight angle) are more or less frontally immune to the IS unless you fire gold. Even tier 8 heavies with fairly weak hull armor such as the Emil can be difficult to penetrate. Even the O-Ni, a same tier heavy, is nearly invincible to the IS if you fire silver rounds (sure, the mini-turrets are technically penetrable, but you will never hit them because of the next problem). With gold rounds, this situation improves somewhat, but plenty of tier 8s and many tier 9 heavies are still very hard to penetrate, and you still pretty much have the worst gold rounds out of all tier 7 heavies (while the O-Ni's gold rounds have 2mm less pen, they are AP instead of APCR, which probably makes up for that).
The gun handling is abysmal. While the IS's dispersion stats are slightly better than those of the T29, its accuracy and aim time are significantly worse. Your shots frequently miss, and because your armor is made of cheese, you usually cannot take your time to aim carefully. If you stop to carefully aim at your enemies' weak spots, they will simply snap a shot through your horrible armor and then drive off. Because of this, even when fighting tanks whose weak spots and LFPs and such you can technically penetrate, you are likely to bounce many of your shots. This combination of bad armor and bad gun handling is probably what makes this tank so bad for me, as I got a 52% win rate in the KV-3, which has the same gun with the same penetration (but which can actually aim its shots when hull down or sidescraping).
The IS cannot hull down. Just to be clear, this is not due to the gun depression. While more is always better, 5-6 degrees is enough for most hull down positions in the game. This is because to hull down properly, you either want good turret armor (so you can sit still and aim) or good gun handling (so you can pop a shot off and then drop below a ridge before your enemies react). The IS has neither of these, which is a huge disadvantage considering that many maps revolve around heavies sitting in hull down positions trading shots.
The IS is not that fast. While it is fairly mobile compared to most T7 heavies, 34 kph is nowhere near fast enough to be considered a true heavium. Additionally, I remember feeling that the IS handles somewhat sluggishly, though that is just a subjective impression. If you are bottom or middle tier, you can try to go with the mediums, but you are still considerably slower than them (and plenty of tier 8 mediums can club the shit out of you, either because they have enough armor to defeat your horrible gun or an autoloader).
It is not usually possible to flank people. People love to say "just flank them!". However, flanking is difficult even for mediums and lights due to how the game works. Most maps consist of a few corridors with some open areas covered by snipers in bushes, so there are usually few ways to get around your enemies until the late game, when many tanks have been destroyed. Given that you are slow compared to lights and mediums and that you have bad camo and will be spotted easily, you usually won't have the chance to do all that much flanking. Additionally, many of the tanks you have trouble penetrating are almost as fast as, just as fast as, or faster than you, which makes it challenging to flank them.
Your vision is horrible. Not only is your base view range of 350 m rather low, but because your gun handling is poor, it is difficult to justify putting coated optics on. While view range is not the most important stat for a heavy, it still helps greatly in a variety of situations (spotting snipers who don't know how to use bush mechanics, doing well in endgame situations, spotting your own targets in mid to long ranged fights for more exp, catching people out when making a "hardcore pressure", as Skill says, etc). On many of my heavies, I have optics mounted and have recieved a decent amount of patrol duty and spotter ribbons. This is not the case in the IS.
You aren't that good at flat ground fighting. Sure, all in all, the IS is probably better than a T29 in a fight on flat terrain. But if two equally skilled players in an IS and a T29 do a corner fight or something like that, the most likely outcome is that the T29 ends up dead and the IS ends up as a one shot. And given the horrible gun handling of the IS, it is not too implausible that you could miss a shot (or have one go up and hit the T29's turret or down and into its tracks or into one of the spaced armor pieces on the hull) and end up losing. On the other hand, if two equally skilled players in an IS and a T29 fight a hull down battle, the T29 will probably kill the IS without taking any damage at all (technically, the IS can overmatch the T29's turret roof, but given the IS's horrible gun handling, doing that at reasonable distances is very difficult, especially if the T29 moves around. Also, if the T29 is using any of its gun depression, then the turret roof becomes basically impossible to hit).
I find it difficult to have a decisive impact on games. In WOT, heavies are typically the most influential class, as they can push through a flank with their superior health pools and armor. However, the IS cannot really do this, because it is pretty much inferior to every same tier heavy (I know about the black prince, I had a significantly higher win rate in that because it can actually hull down and the accuracy is decent). So if you end up in a top tier battle (often 3-5-7 or 5-10 or something), in which case you are usually one of 1-3 heavies on your team, you have 2 choices: you can go to the heavy flank and get clubbed by a T29/Tiger P/O-Ni/etc, or you can go to the medium flank and let your heavy flank get overrun (because you are down one your few top tier heavies).
As a result of all this, I found the IS an extremely difficult tank to play. While it is okayish at flat ground or corner fighting and extremely good at clubbing lower tier tanks (as its armor and penetration are serviceable against them, and its high-alpha, high-dpm gun devastates them), it does very poorly at actually fighting same and higher tier heavies in real world conditions, and thus not a very good tank in my eyes. While I have a 52% overall win rating, I had a 43% win rating in the IS for most of my time playing it. Towards the end of my grind, I began loading more gold rounds (50%) and did a bit better (raising my win rate to 46% and getting 1 mark of excellence), but the fact that it needs gold to perform well is not a good thing (given that I fire gold extremely rarely in most of my tanks). In contrast, I got a 58% win rate in the IS-3 and a 59% win rate in the T-10. The combination of speed, alpha, penetration, turret armor, and good gun handling found in those vehicles simply made them much easier to play (and I rarely fire gold in them). On the whole, I found the IS an extremely frustrating tank to play, and think it is extremely odd that there are not more people complaining on the internet about how bad it is (given that I often see people complaining about bad tanks such as the AMX 65t, TVP VTU, IS-6, etc. on the internet).