I wouldn't say it's terrible. Instead I'd say it was deeply flawed and I found it profoundly disappointing.

SPOILERS HEREWITH

Also, big brain dump.

The fundamental premise makes no sense --- the amount of energy needed to blow up the moon in the way described is about 10^29 joules, if you do the maths, and even the sun takes six minutes to release that amount of energy. It's not survivable to be in the same solar system as that, and I'd be unhappy about being in the next solar system; I'd expect some high-fraction-of-the-speed-of-light fragments coming along fairly soon. The first question any sensible physicist is going to ask after seeing the moon explode into fragments like that is 'why hasn't half the Earth been vaporised?'

(Not to mention that there's no mention as to what the fragments of the moon are orbiting around.)

The research is poor. When the Hard Rain comes, anything in low orbit is going to be toast, because all the bolides are going to have to pass through there to reach the Earth. They literally build the Cloud Ark in the worst possible place. (Why not boost each piece out to geostationary orbit, or better, the same orbit as the Moon but on the opposite side of the Earth? Any fragments you meet there will have very low relative velocity, so you can assemble the habitat there in safety.) Likewise, the entire concept of the Cloud Ark can't work --- it relies on burning thruster fuel all the time, and there is no way to replace this. It's fundamentally unsustainable.

(Also, it suffers badly from Darlings: this is when an author discovers a Cool Thing, and is determined to use it whereever possible. In Seveneyes' case, it's whips. When an editor tells you, kill your darlings, this is what they mean. Unless it's important to the plot, this sort of thing is simply an indulgence and should be removed.)

The plot is... very telegraphed. Very, very telegraphed. He made such a big thing of the mines and the submarines that it was painfully obvious they were going to important later. Indeed, in the last section I found myself saying, "yes, all right, the people from the mines are coming up, JUST GET ON WITH IT AND MEET THEM". And then when the ex-President shows up at the very last moment before the Earth gets destroyed, the entire remainder of the first-section plot flashed before my eyes, and it wasn't a good sight. (Particularly after the promising beginning when he opted out of political soap-operaing entirely.)

Then there's the weird tonal shift half-way through the book, when the cannibals appear. At that point, there is no possible way any of them can survive; they simply don't have the population any more to maintain the skills to run the equipment. As Stephenson says, they live in a high-tech incubator; they need people, lots of people, because they need knowledge to make everything run. With only a double-digit population they might as just give up and die. Instead we take a hard left into some weird metaphysical area, and then whats-her-name gets her magic gene machine out and... I don't know; I had no idea what he was trying to do here.

Then there's the last section, set in the far future. We get a bit of closure from finally seeing the other end of the submarine and mine plots (yeah, and what happened to all the other submarines? And all the other mines? Oh, and his explanation of why the mines couldn't expand doesn't make sense either), but it opens up lots of other plot threads, and seems strangely disconnected from the rest of the book.

As you can tell, I feel quite strongly about Seveneyes; parts of it are really excellent, especially the first third; so much so that discovering that the whole thing was just sort of... incoherent and rambling... came as a horrible disappointment. I don't know what the book is trying to say, or even if it's trying to say anything at all.

Oh, well. I'll just have to go reread Anathem now.