It was good. Not as great as Casino Royale, but only in the sense that it isn't as full-fledged a film in its own right as its predecessor. You'll probably be completely lost in Quantum without having seen Casino Royale, and will surely be missing whatever emotional power the film tugs at with its constant references to Bond's ill-fated love for Vesper Lynd. It exists mostly as the epilogue to Casino Royale, but I still found it fun, exciting, and deep enough (in a rather disposable pop culture sort of way) to sustain itself.
While I very much liked Casino Royale, I remained concerned that this reboot of the Bond franchise had yet to deal with the central problem afflicting most spy thrillers these days: the Cold War is over and ignorant religious nutjobs in caves don't make nearly as interesting devious masterminds as our mythical versions of the KGB did back when Bond had a Scottish accent. So, taking its cues from Jason Bourne, Quantum pits Bond as much against the Americans and his own government as it does against a not-too-original alliance of a tinpot dictator and an omnipotent secret cabal, finally making a Bond film whose subtext doesn't play as an anti-Communist screed but as something matching modern-day concerns. Indeed, if Quantum is anti- anything, it's anti-globalization, with the real villain being the influence of amoral corporate interests on government intelligence policy.
Yet Quantum of Solace is still a Bond film, and is really about an ultra-cool badass playing the "kiss-kiss, bang-bang" game. Lacking a new Vesper for James, there is less "kiss-kiss" than in Casino Royale, but the more "realistic" subtext saves the film from being less human because of it. The film is also tighter (though just slightly) than Casino Royale, with nary a breathless moment between it's numerous fight scenes, yet more than enough plot (it starts to feel like Syriana at times) that it doesn't feel like one damn battle after another (i.e. The Matrix sequels).
If there is any real criticism, it's that Quantum ends too abruptly. Having set up so many plotlines, many are left to be tied up unsatisfactorily in dialogue exposition after all the explosions have burned themselves out. While it makes a nice coda to Casino Royale, it's nothing more than that, but it does a very good job of laying the groundwork for where this new Bond is headed.