I'd say the build quality of this lens is "L+". You could batter somebody to death with it and it wouldn't harm the lens one bit.
Optically, of course there are going to be compromises when you're designing a 10x zoom. Of course it's not going to be as good as a 24-70 or a 70-200. But it's certainly not bad. Canon haven't tried to cut corners when they designed this one. The only significant gripes reported by reviewers on Fred Miranda are the weight and the cost.