I also have my copy of Sins of a Solar Empire, which I had before, but never had the opportunity to fully explore (its incredibly complex). I'll eventually try playing online, but not until I'm sure I've sufficiently learned the ropes (IE, I'm too chicken to play against people that primarily play online).
Please note that my experience with RTS games is very limited. Played WC, WC2, SC, SW:GB, and DoW1, and of those DoW1 was the only one that had ever captured my interest for any significant length of time. In fact, as much of a sci-fi fan as I am, SC would probably be my least favorite of that, admittedly, short list.
Now Sins of a Solar Empire has been out for awhile, so I don't think I can say much that hasn't been said in full reviews elsewhere. I'll just say that this is while the primary method of winning is to build a fleet of ships to level your opponents planets, getting to that point of attacking and winning can take awhile and can come about in several different ways. You can build up your warchest through diplomatic means by controlling large numbers of colonies, or you can build up to your ship limit early and attempt a bumrush (or fight off the enemies' bumrush). Either way, even against easier AI, the game will go for awhile. This is a very methodical game, with a large number of variables to deal with, from upgrades, build-limits, to pirate raiders that all players pay to go against the other side.
In order to win, you will generally be dealing with leading a large number of single ships, including large capital ships that level-up as they fight (and survive), each with different capabilities, against enemies with their own ships that are analogous to them (in terms of base stats). It can get pretty hectic when large numbers of these ships start going at it. While each of the three races have (as I said) ships that are roughly analogous to eachothers (and researched upgrades to go along with them), the devil is in the handful of special abilites that are unlocked by research (for the tier 1 and 2 ships) and leveling (for the tier 3; IE capital ships). Unfortunately, I haven't played enough to get into games where most of these abilities come into play, or even need to be researched for that matter.
I'll gave the game high marks, no doubt. Its well crafted, well thought out. But its definitely not a game you can just jump into, ESPECIALLY if you haven't played RTS games before, and it lacks easy pickup and play capability. So far I'd have to give pickup and play ability to (of that list) to DoW1. BUT, I am biased, so bleh.
I'd attended to talk about DoW2 or Demigod originally, but I guess those will have to wait until later. If you want to know more about Sins of a Solar Empire though, I strongly suggest Googling it and reading articles and reviews by people a little more knowledgable and concise than myself. :P