To give you an idea of how popular it is here, there are four PBEM Dominions 4 games currently in progress in the Play With Your Buddies subforum, another that just ended after less than 30 days (403), and another (406) that will be starting soon and already has 12 people signed up for it.
There's a huge manual in pdf form (also orderable in dead tree form), clocking in at around 400 pages (IIRC) including informational charts and so forth. It's entirely possible to just start a singleplayer game with a random nation and make a pretender on the fly without knowing what you're doing, just to see how it goes and learn from the experience, but you wouldn't want to do that in multiplayer, obviously, and generally folks will study a faction's units and their capabilities before deciding how to design their pretender. You can also make a pretender, and then test it out in SP. You'd want to have some understanding of the gameplay while designing a pretender, ideally, but you can get that either from reading the manual, or from skimming the manual and then just making something and trying it out in the game (and for me at least, experimenting is an easier way to learn than trying to read, understand, and remember a 400 page pdf).
It's also much faster than civilization multiplayer in terms of what you can do in one turn.