Bikari: I'm not incredibly deep into them, but I'll offer my experiences:
KV-1: Hands down the best T5 heavy and a credit-printing machine. Not much to say about it, apart from noting the autobounce plate on the forward armor, as that is carried over to a lot of higher tier heavies. The basic lesson it teaches is to always keep your hull oriented towards the enemy line, as the sides are fairly weak and unsloped. The top gun is damned good for the MM it gets, and the frontal armor is also very good. You can bounce shells from T6-7 heavies if they hit the right places.
KV-1S: Crap until you get the long 122mm. Once you do, it makes for a pretty good flanker (as it has a decent turn of speed, comparable to a slower medium) and sniper/peeker, thanks to the longish reload. Like all of the T6 Soviet heavies it has the autobounce plate; also like them, it has fairly weak side armor.
KV-2: Garbage until you have the 152mm derp. Once you do, it's a god-tier support tank. I've had plenty of times where I one-shotted T6 heavies with side shots, and it is pretty much auto-kill for T4-5s, apart from some of the T5 heavies. There's nothing quite like seeing a T8 heavy running away from you, either. The gun mantlet occupies most of the front of the turret and can bounce or eat almost anything you face, and the frontal armor is still good, but the sides of the turret are massive, vertical, and thin, as is the rest of the hull.
It's very much a support tank; I once managed to pull off a win in a 1v4 situation, but that was only because the enemies were idiots and came at me piecemeal. You're looking at an obscenely long reload time even with a rammer and 100% crew, so you really aren't suited for front line brawling. People will yell at you for sniping with it; don't listen. If you can lead your shots properly, you can pop enemies from very far out, provided you let the gun aim completely. Hide behind cover and bigger friends, take your time, and you'll do very well.
IS: Essentially the KV-1S mark II. Better armor, a bit bigger, and that's about all. The second best gun for it is the long 122 from the KV-1S, and the top gun is the same gun with a very slightly faster reload. Now, granted, it's still a decent gun for this tier, but you'll do better playing support/sniper in T7-9 matches. Also note that if you've elited the KV-13, the top gun for that is one of the available weaker guns for the IS, and it makes a decent stopgap until you get the upgraded turret and mount the long 122.
I haven't played the T-150, but from what I've heard it's essentially the KV-1 but more, so an all-around brawler.
Basically they're very good tanks if you play them properly, and they're tough enough to take a bit of punishment if you screw up. If nothing else get the KV-1, as it's pretty much an assured minimum of 10k credits per match.
Also, if it's worth anything, three of my four highest (base) exp matches were in the KV-1S, KV-1, and KV-2 respectively. My match with the most kills (
was in my KV-1, and I'd estimate that around 2/5 of my Top Guns came from Soviet heavies.