If you're a SupCom player, think of this as pretty much your dream patch for the game, but with the unfortunate but understandable addition of a price-tag. If you're an RTS fan who hasn't played SupCom yet, this is without doubt a better place to start.
It's more polished, it's more varied and, with the Seraphim's big'n'shiny toys in there too, it's wargaming on the most jawdropping scale there is. It's also everything that turned a lot of people off SupCom intensified, and I think that's a bit of misfire if it's serious about winning a new audience - which is why this scores less than the first game did.
Why else would it be standalone? Well, perhaps so that SupCom players have to cough up more groats than they would for an expansion disc - but that's tinfoil hat territory. If you're even slightly interested in real-time strategy, you owe it to yourself to play this - but leave that merciless, exhausting campaign well alone until you've made yourself into a war-god with skirmish mode.