The graphics will appear grainy, especially if you've played, for instance, Warcraft III.
But I and many others would say that it has no equal in the RTS genre, in terms of game mechanics, balance, fun. I'd say it's Blizzard's masterpiece, thus far. It has a vicious learning curve, but I still play it to this day.
Make sure to click each unit repeatedly to hear all the different things it will say, by the way. Particularly the firebat, and the terran siege tank.