(The Goliath can be found in the Elemental Evil Player' Companion PDF here: http://media.wizards.com/2015/downloads/dnd/EE_PlayersCompanionv2.pdf )
I don't massively dislike Goliath's and have considered playing them but...
Mechanically they're not astounding
Powerful Build and Mountain Born feel a little bit irrelevant
+2 STR and +1 CON = Not bad, but Half Orcs provide that and Mountain Dwarfs do better with +2 STR, +2 CON.
Proficiency in Athletics, definitely useful, but mostly to be thought of as "an extra skill" since if you're picking Goliath as a race, chances are you'd take Athletics anyhow.
Stone’s Endurance said:
You can focus yourself to occasionally shrug off injury. When you take damage, you can use your reaction to roll a d12. Add your Constitution modifier to the number rolled, and reduce the damage by that total. After you use this trait, you can’t use it again until you finish a short or long rest.
So in terms of pure mechanical benefit I think I'd rather go for Darkvision, Relentless Endurance and Savage Attacks from Half Orc instead.... or Mountain Dwarf.
In terms of background, character and theme...
Most D&D races are fundamentally "humans in prosthetic and funny hats" with cultures that aren't really very alien at all.
Now this actually mostly serves to make roleplaying a lot easier rather than working out what an levitating jellyfish without a conception of an individual would be thinking in any given moment.
... But Goliaths do seem particularly indistinct from a human tribe who lives at the top of mountains and value physical competence and competition.
Arguably this isn't a awful thing, as I tend to prefer playing humans so I don't feel like I have to adopt or consciously discard that culture baggage that comes with the race.