Is regigigas good?
I checked the pokemon go hub database, and says that it has a final max cp of 4346, while it puts regigigas at the highest max cp of any legendary, it also has a respectable 287 attack stat. But when u turn to the fast and charge moves, it actually has no STAB fast nor charge moves. So this is the question: is regigigas going to be relevant?

Answers
Zen Headbutt/Avalanche makes it a bulky option against flying dragons like Rayquaza, but not the best option considering Mamoswine, Weavile, and Glaceon come out this generation as well. If Rayquaza raids return after Gigas's debut but before the three ice types mentioned previously it could be useful but at that point I'd rather sink the rares into an Ice Beam Mewtwo.
Regigigas would make a pretty damn good gym defender, if Niantic ever lets us stick legendaries in gyms. As a raid attacker, it could be decent in some niche scenarios (like Rayquaza) as others have mentioned, but that's about it. There are also better options for generalist attackers and gym sweepers. Maybe it will find another niche once we get PvP?
It is not bad, a decent one when use as an attacker especially if you are not looking to shortman a T5 raids.
But for trainer that already play for a while, they shoud has a good counters for the T5 raids ready that performs better than Regigigas.
And as a legendary, the cost to make a battle ready one is expensive.