I bought Sins of a Solar Empire in February 2009 for $20. I had a lot of fun with it. Not long after, I picked up Entrenchment for $10. Had fun with that.
Then Diplomacy was coming out, and there wasn't a whole lot of information on it, but I thought, "Hey! I'll preorder it anyway." I've only played diplomacy (or Sins, for that matter) a handful of times since then.
Now it sounds like the same old story. Another relatively small tweak to the current game which will change strategies but likely not revitalize the game all that much.
I'm not going to make the same mistake I did with Diplomacy and preorder when there's little evidence there'll be anything except what has already been announced, which isn't really that much larger or gamechanging than what's already come out (the two smaller expansions). I figured back with Diplomacy that the small amount of information was just because they weren't ready to talk about it. Turns out the short list of new features really was all they added.
Don't get me wrong. I like Sins (though the novelty has worn off.) I've spent many hours conquering the stars when I ought to have been doing other things. But I've gotten the feeling that Ironclad has gotten a bit... creatively stale. Sins doesn't seem to be changing a whole lot, but there seems to be quite the grab for cash with Rebellion. $40 is just too steep for what boils down to another mini-expansion. $40 is buying a completely new AAA PC game a bit after it comes out. Deus Ex (amazing game!) just came out, and now you can get it for $40. I also just picked up another indie RTS (Achron) which has some really crazy features. That complete game costs only $30. Rebellion is essentially a 3-year-old indie game priced like a current AAA title.
Who but the hardcore-est fans (i.e. play it more than once in a blue moon) would get Rebellion, other than people who haven't sank the money in for Trinity (either $40 as I did, or $20 with the bundle)? It's just not that much new content. The bang for the buck is just bad.
If you want to drop $40 (or $30) and get it, I have no problem with that. Ironclad is a small studio and I'm sure they need the money since the initial sales of Sins are probably down by now. But chances are, I won't be getting Rebellion unless the price drops. Significantly. Or they announce that their list of new features was only a partial one, and that this is actually the large "stand alone" expansion they claim.