How good is lugia for gym attacking and how good is it in general?

In terms of attack I'd say a big no.
The type of Lugia is Psy and Fly. There are plenty of better attackers in these regards.
Alakazam and Espeon are 2 of the best attackers, (ranked 2 and 4 respectively, Lugia being ranked 60) and they are both Psy. They can also learn the move Future Sight as Lugia but with much higher damage output. And don't forget Mewtwo the most powerful poke in terms of attack may be released soon.
For Flying, a Dragonite with Hurricane can offer much more powerful punch than Lugia does.
The only Lugia offer for raid battle is perhaps his bulky defense and stamina which he can last longer than the aforementioned poke. So I'd say save your dust and candies for Mewtwo
Hope it helps :)

Well, I had only succeeded in solo machamp once before lugia, now done it twice since with a Lugia in team powered up to level 30. No regrets, depends what you want. He looks really cool too!

Maybe don't power him up yet, but hold him tight.

Raiding is a thing atm but I don't think it's going to sustain the game long term like a decent gym system can.

Lugia would have been amazing in the old gym system, or in any gym system in which strong pokemon actually matter.

Got 97,8 Lugia so i "had to" max it. You can sweep a full gym with a +3500 Lugia without healing, sometimes multiple rounds. Definitely not the fastest gym attacker, but not slow either. Sky Attack is really fun to play with, really fast. If you like to save potions and if you like the pokemon, definitely buff him up. If not, dont. Lugia is not "needed" for anything

Give and take, maybe just think of him like a slightly better flying "snorlax"