I wouldn't jump up and down too much about that 80% rarity factor. I just bought 2 KV-1S for a scenario in July '43 and rarity was only 30%. But they're still quite expensive and on that scenario blew my entire budget for armor (couldn't sqeeze anymore into the armor limit).
Now I'll be the first to admit these are nasty tanks. Becket certainly knows this after I stomped all over his armor with them. The only concern I've seen using them so far (I admit it, I'm a noob so I could easily be wrong) is that they don't seem to be very easy to even get a track hit on and immobilize. I'm not sure how frequent something like that is on other tanks, but Becket has gotten 4-5 track hits on one of my KV-1 tanks in my game with him and it's still moving. Though I think his track hits where from his silly little AT Rifle team that I was ignoring while I chased down his last PzIII so probably not damaging enough.
The other thing you need to remember about them is compared to your Panzers they're SLOOOOOW. Super slow, they don't turn fast, thier turrets aren't fast, nothing about them is quick.
They're tough tanks, espicially early on in the war, but they're not invincible.