I can't comment on single player, we always played over LAN (I wont bore you with low opinion of no LAN option).
The game feels like Starcraft without the Broodwar expansion added to it. Units types feel limited and linear.
A lot of the abilities have been removed like Red Archon Maestorm and Mind Control (no great loss), no scouts or basic aircraft for protos, Zerg Red Acid and Dark Swarm, No Zerg evolution of aircraft at all, No Arbitor unit full stop (mothership has similar abilities, but limited to one). No Hydalisks evolution to the underground spikey version, which combined with the now hopeless Zerg turrets mean that Zerg get pummeled every time we've played. The protos have lost so much as well.
The only strategy seems to be brute force now, and we've found the humans simply walk through the other 2 teams without problem Small groups of armoured humans in a mixed formation seem to walk through most situations. They also haven't fixed the AI pathfinding problem from the original, where a human places Supply Depots at the entrance to a cliff and the units attacking line up to be slaughtered or wander round the map - in fact they've made it worse, cos Supply depots can now lower to act as a gate, giving the humans no problem being blocked in.
Skirmish is laughable against computer, as should you build up even a small army, you will walk through most AI levels and the Insane doesn't give you a chance to start the game, let alone build a base.
Buildings also collapse so quickly, it now reminds me more of C&C Red Alert and C&C 3 Tiberium wars.
There are some clever new additions, but the game feels very limited and I think it's been completely over-hyped (and I've been playing the original over LAN since it came out all those years ago). If this didn't have the Starcraft badge on it, I say the reviews would have been less favourable.
Graphics - Humans and Zerg are nice looking, but the protos look cell shaded and not very futuristic - it doesn't suit the style of graphics used on the other 2 teams (my machine has everything set to ultra).
The Specs required are at least quite low. I have an AMD x2 6000+ with cheap 9800GT graphics card and everything can be set to Ultra. By modern standards that spec is nothing special. Supreme Commander 1 seems to struggle a little with big maps on the same spec.
This game is very average.