The plot is... meh, but the gameplay seems much improved, and the AI is very good. In SC1, you could eaisly tell you were playing a computer. If you attacked on guy in a group, the whole group would go "we're under attack!" and charge after you. When the computer decided it was going to attack, it didn't matter if the front of your base was lined with 25 cannons--it was going to attack, succeed or fail. When it decided it was going to do dropship-attack, it didnt' matter if they were sending 25 overlords and you had one missile turret... the overlords would get hit by the missiles and go "oh shit, he was ready for this one" and run away.
Not so, in the new one. I've only played the computer on normal in head-to-head games (which is about where my skill level is), but it seems like they have an internal meter judging the relative strengths of forces. A zerg troop ran into my zealots, and as soon as they started losing, the computer turned around and ran away, trying to save the rest (about 3/4) of it's army.
It's very cool, overall. I like it a lot. Just not the campaign story.