I think Delphi is okay if you're doing a low age strategy since it's so easy to build and level up, and it has a usefully small footprint. And it's okay as a starter GB for newbies. Otherwise it's pretty weak.
My post on the front page is in some dire need of updating, maybe I'll finally finish working on it eventually. But my views on the GBs introduced after I posted it:
Kraken: Generally good for anyone, though it's somewhat combat focused.
Blue Galaxy: Build this only if you have a lot of buildings that make multiple FP. Originally it was best for people with lots of Terrace Farms or Indian Palace sets, but a player with a good collection of leveled up event buildings will probably find it beneficial. Also potentially useful for WW farms.
Terracotta Army: A must have, IMO. Boosting all attack and defense if useful even for someone who doesn't fight a lot.
Himeji Castle: Excellent for anyone fighting, but probably not useful for a pacifist city. You'll want to level the attack boosting buildings first though.
Virgo Project: Doesn't really activate enough times to make it worth it, except maybe for GE fights. Not enough uses for any serious GvG, and won't work in PvP. Battlegrounds might make it more useful, but we'll have to see how the meta develops.
Star Gazer: Good for people who do a lot of negotiating or trading. Probably good for people who farm goods for the guild. I think it makes refined goods (don't feel like checking right now), so it becomes more useful from PME onwards.