I've played the game since not long after it came out, bought the expansions as they came out. Played quite a lot of hours, don't even know how many, but at least 30-40, probably more. With that, if I had to face the idea of buying all of that in one go, I would not do it.
The game is enjoyable, especially if you like space strategy type games. It ostensibly allows you to focus only on exactly what you want to deal with, though the AI control can be noticeably sub par to what any partially competent person can manage. When I get bored sometimes, I will build up a massive powerhouse empire and then just switch to full AI control and time how long it takes for it to be run into the ground. That said, as long as you keep a half eye on what the AI is doing and veto the dumb decisions, it is workable and can make the game more enjoyable.
AI shenanigans aside, it's a reasonably well made 4x RTS. The interface has a bit of clunky to it, but it's really not that bad; I've always been able to do everything I wanted to without wanting to stab the developers in their faces. I do find the ship design area a bit annoying in some respects (what upgrades you can use can feel arbitrary at times, since it's reliant upon the huge number of semi-rare resources you may not have), but that's largely because I don't pay much attention at all to what my colonies and constructors are harvesting. It does offer an export/import feature for ship designs, which I naturally forget to use every time. Research is nothing new, but it does have 3 'areas' you can research in simultaneously.
Diplomacy never struck me as very well developed. It's quite simple to play the AIs off of each other, and to act as a technology broker. Maybe I just never gave enough of a damn about it though, someone else may have a better impression.
At the end of the day though, it's fun, and the combat is great to watch. I would buy the first game and see how you like that. The expansions fixed some of the bugs and added a few features, but if you hate the first game the expansions won't salvage it for you. If you DO like the core game, then the expansions will be worth picking up once you've determined that.
Oh yeah. Zooming pisses me off, it's not smooth.