It's got that big, derpy naval gun, a good health pool (700 HP? On MY T6 TDs? That's only ~110 less than the T6 Russian heavies!), and surprisingly good armor from the front, as well as a strong engine and a good hull traverse speed.
The gun, specifically: that 130mm with standard AP shells has 196mm of penetration and an average damage roll of 440, as well as a 3.75 shell/minute ROF. The 122mm that the SU-100 gets, at the same tier and as a non-premium vehicle, only has 175mm of penetration, 390 average damage, and 4.69 ROF.
Actually, the SU-100Y's 130mm gun is hands-down the strongest gun any T6 TD gets in terms of combined penetration and alpha damage. The top guns on the ARL V39, Churchil GC, and Nashorn have 7-18mm more average penetration with AP, but half the alpha damage. It doesn't get premium matchmaking, but it doesn't need to -- you can punch the ticket of just about anything you'll encounter; with a half-decent angle it can punch through any spot on any T8 heavy that isn't the turret front of a fully upgraded T32, IS-3, or Chinese heavy. If you're anywhere near top tier, you're going to be one-shotting people left and right.
The armor's not fantastic, but it's not terrible -- 60mm all around, no real weakspots, little good sloping. You've got that KV autobounce plate and your engine deck might as well be too, but that giant flat-walled barn is easy penetration for anything that can beat your armor if they get around you. That said, the gun mantlet is thick, and those 45-degree front cheeks are surprisingly bouncy. If you're caught in a situation where you can't pull back or sidescrape during reloads, just flail your front around.
It's one of the few premiums I've bought, and I don't regret it. It's second only to the KV-2 for mid-tier derpiness.