Having modded this game extensively, especially during my work on the Vector Prime mod, I know a bit of the ins and outs of the game (not as much as the other members on the Vector Prime team).
In short, the game can be considered a realtime lite 4X game. You start off with some ships, some planets or just your homeworld depending on the scenario. Your goal usually is to destroy all enemies (other victory conditions are entirely possible!). Allied victory is allowed on most maps. You colonize/conquer/glass planets, mine resources for money, engage in commerce by setting up trade routes to gain money, tax your population, research technologies, build ships, explore unknown systems and watch epic explosions unfold as your military base gets whacked by your opponent.
The game does have a wider playing field compared to homeworld 2 (haeg maps can go up to 12 solar systems with various planets in them) but has less ships in the battle. Planets are your main production centers, which will provide a healthy dose of tax (if you did things right), production capabilities of ships, bases and buildings and the research capability is directly tied to population. High science output means less production speed.
The game's economy has only 2 resources: Research Points and money. Money follows streaming economy principles in which sinking more engineering power on your newest battleship will make it roll of the construction line faster, but the money will be emptied from your coffers faster. Money gets deducted during construction, and doesn't require you to have all money on hand beforehand.
Space battles are quite good in this game. Nothing more awesome than seeing a bunch of small fighters launch a rocket volley at that darzok battleship and seeing it break apart and explode in the most visually pleasing ways. Bigger ships do have some sort of subsystem mechanic, but its limited only to targeting weapons or engines (besides the default 'punch holes in the enemy hull till it explodes manner). There are 4 basic types of weapons: Missiles (hard hitting, but good ECM can counter them hard), Proton Cannons (Fairly fast, decent damage output), Quantum (Beware, splash damage!) and Ion Beams (High accuracy. nothing more awesome than seeing your enemy's mining base split in 2 by your nice beam weapon).
The game does have some nasty limitations, both in gameplay and in the modding field. In some areas it can be seen that the game was rushed, mostly due to the lack of a campaign (which legions of iron does have). Also, another issue is the game's maps having ludicrously low unit limits often, which is something which you can change yourself without a headache.
The Solon Heritage does feature some good mods, although most of the old modding community died off after the sudden disappearance of the haeggalaxy community. Still, there's some good mods around, including the mod i worked on with a few others: https://www.youtube.com/watch?v=0BIMDE4_Iks
All in all:
Can you stand somewhat outdated graphics?
Do you like realtime empire building on a small to medium scale?
Do you like epic explosions and plenty of colorful tools of death?
Do you like an epic soundtrack which will make your ears drop off your head and go roll on the floor?
If the answer to that is yes, then by all means, buy it!