Most of my games (well, all except one), I have either had to build a ZPG myself or get one far away. But my most recent game had a few of them rather close.
I now have captured 3 ZPGs, and built 1. I run my whole empire on them
But it is not without a cost, instead of going for the best planets, I formed my core planets around the ZPGs, so I got a few non optimal planets that I had to take. And I only came out slightly a head then avarage. No need to build Mk II and III reactors (yet at any rate), but on avarage the planets I got was more meh.
Imo, they are not overpowerd at all. If you want to re-roll all your maps until you get them close, well, thats your chooise, and not a game balance thing
. I for one re rol maps untiel I see a structure I like, tehn I take it from there. Going into the unknown is more fun I think, sometimes I get lucky, and somtimes the unknwon gives me opportunities with good rewards if I dare take them
Also remember the penalities on them. Loosing one sucks. So having many of them means you need to defend them as well, more so in CPAs. Defending my empire in CPS is not that much fun since Irun the risk of incuring more AIP, and face a sudden energy crises. (when I think about it, I should build all the MK II reactors and just offline them now before I loose teh ZPGs....)
All in all, they can provide nice benfits if done right, but they are not the be all end all.