WARNING - READ THIS FIRST
Several users have encountered a sound bug with the game. This can be fixed, but it may require several steps involving changing game files and mounting ISO's. If you are not familar with doing this, then do not buy the game. Please refer to the forums or PM me if you would like to know more.
Valve - Please don't release broken games or at least provide support as to how to fix them (Vampire: The Masquerade also comes to mind).
Haegemonia is a space RTS that has some similarity to Homeworld, Sins of a Solar Empire and Stardrive. There are some limited 4X elements to the game as you colonise and manage planets, research new technologies, spy on other factions, hire heroes, manage your economy and experience random events. The game is more basic than the others listed. It does not have the advanced fleet management of HW, the diplomacy system of SOASE or the intense micromanagement of turn based 4X games such as Endless Space or Sword of the Stars. In this sense, the game is a jack of all trades, master of none. However, for its time, this was a fairly ambitious attempt at merging several genres - a feat still yet to be fully achieved (Stardrive was not a resounding success). If you liked these kinds of games, but occasionally desire a more casual experience, then you should find Hae enjoyable if not as satisfying as these other titles. If you are new to RTS or 4X, then Hae is a great way to begin learning some of the basics (this was my experience as I purchased the game when it was first released).
Graphically, the game is still very beautiful. The ships especially eclipse the fidelity of HW2 (although the HD edition of Homeworld should soon be with us) and although some of the texture quality is not quite as good as Sins, considering the age of the game, it is far from distracting. The game runs smoothly (aside from the bug above) on the highest settings. The options menu is very limited in this respect, but this again may just reflect the age of the game.
The sound assets to the game are fine, with farily satisfying ship explosions. The music however, is limited, with the only switches being between a "peaceful" single soundtrack to the "battle" soundtrack. It would have been nice to have some variance here with some different "spacey" soundtracks to give a bit more of an immersive feel.
The maps are limited, but there is lots of variation. I would have liked to have seen more maps in general and some more maps that have environmental hazards that add interesting dynamics to the game (for example, there is one map with a hostile Solon base in the central wormhole hub). However, there is an excellent modding community for the game, and I imagine that it is possible to download more maps from there.
As far as balance goes, spying is the best way to get a significant advantage. Your research points are limited (unless you change the parameters to otherwise) but stealing technological prototypes reduces the cost of research. Human's have an advantage both in spying and research, so they are rather overpowered in the right hands. I honestly can't see there being a bustling multiplaying community for the game, so this probably doesn't matter.
All in all, the game is solid if a little limited. It's not the thinking game that perhaps some of its descendents and predecessors are, but if you are looking for a visually impressive more simplisitic space strategy title with a small price tag, I recommend picking the game up.
Edits: Some spelling and grammar issues.