Worth playing if you liked the first two in the series.
This is your standard explore, colonize, research, and then blow up the enemy type game. You can immerse yourself in micromanagement minutiae or just let the computer handle all that small stuff. I prefer to handle all the small stuff myself. Sure, I may build too many mining and bioharvest DEAs on the planets, but I still have fun doing it.
This game can be hard. You're at the whim of randomness when trying to find colonizable planets nearby to your home system early in the game. This can make for a very frustrating game. A lot of your time will be spent looking for planets to colonize and then engaging your enemies and taking over their systems, hopefully.
If you want to succeed in this game, you're gonna have to research like mad. You will need a leg up on your competition and new tech helps tremendously, both for planetary developement and in battle.
There are a few ways that you can 'win' the game, from being elected president of the Orion Senate to discovering all five of the Orion X techs. Then there's always the ol' standby of crushing all your opponnents. In a big galaxy, that can take a lot of time.
The game is very enjoyable, especially when you add a lot of computers players to the mix. Makes for more trading partners and more potential enemies. Just let them colonize some planets and then come in after them and conquer them for yourself.
If you can find the first two games, they are definitely worth looking into. That way you can see the progression of the series. The first is very old school and a classic. The second changed things up quite a bit. The third was a huge departure from the second, but I think it is a very solid game.