The idea behind this novel (a cosmos where the gravitational constant is one billion times that of ours), is extremely interesting. Baxter should definitely be applauded for coming up with something like this and fleshing it out somewhat. Unfortunately, his writing skills are lacking. First, the overall flavor of the novel is somewhat juvenile. Characters are flat but also inconsistent (similar to what one reviewer here said, the main character is a genius leader one minute but an idiot child the next). Second, the plot basically is held together through miraculous happenings.
But, worst of all, since Baxter is a physicist, is that Baxter's physics are inconsistent (i.e., wrong in some places). For instance, the Belt is a linked set of facilities in orbit around a "star" (which is, itself, in orbit around the center of the cosmos, the Core). There's a microgravity field from the Belt's own mass pulling things from above and below. Yet, somehow, the miners drop a chair down to the "star" by cable. Orbits don't work that way. Assuming they could get the chair away from the belt (and a simple push would probaby be enough), all it would do is go into an elliptical orbit crossing the Belt's orbit. To get to the surface of the "star," they'd need some kind of thrust (and I won't even go into how the cable would end up wrapping around the "star" as the chair changed orbits).
Another example from the Belt is when they're trying to deliver a very heavy food machine. The thing is floating above the Belt. That means it's co-orbital with it. The ropes holding the machine break and the thing falls past the Belt, past the star, and down to the Core. Sorry. But since it's co-orbital, the darn thing would just float around there. Baxter uses that co-orbital floating trick later in the book when a couple of the characters float around "above" the Belt until rescued.
There are similar physics problems at the Raft. First, and very obviously, there is a "star" which is "falling" towards the Raft. It stays there for most of the book. But, since the Raft is orbiting the Core, there's no way something falling toward the Core from a higher orbit would stay fixed above the Raft. Since the gravitational constant is so huge in this cosmos, orbiting bodies move VERY quickly. That "star" would be spiralling all over the heavens on its way down.
In another Raft case, some bad people are trying to make some others "walk the plank" off the edge of the Raft. So what? Again, this thing's in orbit. Walk off the edge, and aside from local gravitational effects, you'd just hang there. This is very similar to a point near the end when the people break a big chunk of the Raft off. It goes plummeting "down" and people fall though the hole to their death. Once again, orbits don't work this way.
There are a lot of other lesser things that are wrong about the physics (the atmosphere is in orbit, too -- where's the weather?), but those are the big ones. With the plot and character problems, these essentially make the book not really worth reading. It's a shame, since the idea behind the book is so clever. But, I just can't recommend the book.