My '09 Cannondale 29er came w/ X5 standard and it's durable, but heavy and a bit clunky. (I've since upgraded to X9)
In an age of much better derailleurs (XX, XO, XO1, XX1, X9 and X7), X5 is nothing special and entry-level at best. Shoot, even some of the 'best' WalMart bikes come with X4 rear derailleur.
As far as "cheaper components elsewhere", the Giant is loaded w/ entry-level-spec parts: Suntour XCT fork, Tektro brakes, Suntour cranks/bb, mixed-brand shifting, etc...and while none of this means that it's a bad bike, the parts spec is going to become more and more evident the more you ride it and as you improve your riding abilities, but for sub-$600 it's spec'ed pretty normal.

As always, I recommend you test ride whatever you buy before you buy it. A good local shop should let you do that in order for you to make a wise decision.