In my opinion, Cannondale is the undisputed king of building Aluminum frames, the CAAD-X frame is derived from their legendary CAAD road frames. (Caad is an acronym for: cannondale advanced aluminum design, and it is true if you ask me.)
I have owned the CAAD10 road bike frame since 2011 and it is so smooth i often ride that instead of my other Carbon bikes that cost 2-3 times more.
The only minor thing i don't like on the CAAD/CAAD-x is the BB30, if it had a threaded BB that would be 100% perfect in my eyes....but the BB30 isn't a deal breaker, i am just not a huge fan.
I will never consider an Aluminum bike from another manufacturer.