Pacts, How useful are they?

it quite honestly depends.  Ill tell you one thing though, the tier 7 and 8 pacts are high levels of rediculous, and the first few ones, like the research pact, and resoruce pact, and trade pact are great.

THe larger the map, the better the chances of pacts beig usefull. in, say a 2v2, if you and your ally dont have 5 planets each, then you polly should not be worried about pacts, however, if you each have 10 planets, they jut might be usefull.

in larger games, if your stuck in an econ spot... getting a supply pact to a front line fighter is gamechanging. (imagine going from 520 supply to 1420 supply without an increase in upkeep. ya. exactly.)... and if you can get away with it, is something you need to be going for very early on. (I mean, getting a trade pact and a metal pact with anyone is going to significantly increase your own income, and considering you are already in econ spot...  if your in econ spot, going for the endgame civic techs, like RA, pervacive econ, and the pacts, is now definately worth it.

but, if you in a smaller map, pacts (and civic techs in general beyond tier 2) are generally not worth it, and you should be investing in fleet and such. 

The pacts are SICKLY POWERFUL, at least some of them!

The most awesome include, but are not limited to TEC's supply pact, Advent's shield pact (90% shield mitigation caps pl0xty), Vasari structure pact (+25% health to everything, almost like doing the whole hull upgrade research line all at once).

If you're in econ spot and you have a Vasari ally, I strongly suggest you convince him into sending a few envoys to you with their (tier 4?) Mutual Threat ability. If you're Vasari, you should send these to every factory planet of your allies.
Why? Cause triple resource extraction rates and triple production rates FTW.
Seriously, assume you have four extractors at 80% allegiance (plus some basic research) - that's about 1.2 resource/second. With mutual threat it (temporarily) becomes 3.6 resources per second. 2.4 extra resources/second means roughly ~ 5-6 extra credits per second (if you simply sell it on the black market for a mediocre price) and this is ~2-3 trade ports (unless you have a 30-planet chain). Great stuff imo! Coupled with some other tricks (resource pacts, resource extraction research) you can really rack up serious income.

Altogether, the diplomacy research tree is capable of empowering you very, VERY much.
Use it.

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I suspected it was very map dependent. But I will defiantly explore that tech tree more cause wow as explained those are some amazing bonuses!