It's taken a while, probably because of the E3, for the first review of Superstar to show up. Although GameSpot probably isn't the very first, they have the first published review that I found. Here's a bit they say about the game's stability, which has yet again gotten worse:

"It almost seems pointless by now to mention that The Sims: Superstar, like all the other expansion packs, doesn't bother to address the annoying problems that have plagued The Sims ever since 2000. The game's scrolling camera is as sluggish as ever, the pathfinding is still unreliable. And as usual, your sims will still occasionally "forget" the instructions you give them. This can be especially frustrating when another celebrity sim tries to speak to you to increase your fame while you're in the middle of something else. In these instances, the celebrities will seemingly "forget" to speak with you the instant your sim becomes available. And unfortunately, Superstar shipped with a few bizarre crash problems. At least some of them can be fixed by adjusting your AGP graphics settings or running the game in a window, though hopefully Maxis will release a patch for the expansion."

Besides the technical issues, they find it a good expansion pack that's worthwhile. It adds a lot of nice new gameplay. The 8 out of 10 the game got is definately not bad. You can read the whole review here. This review also means the opening of a new review index, which you can find right here.