It was a great buy when I bought it years ago, because UQM didn't exist back then.
Now, if you can get it for free, it'll be barely worth your time.
It's a really mixed bag. Much of what you do is simply boring, repetitive colony management. The tasks they took away were moderately interesting, and the tasks they set you are just dumb.
You occasionally get into a fight, but aside from the trivially easy daktaklakpak... not much. The balance of the ships is poor to awful, partly due to inexcusable bugs, and partly due to AI that's smart enough to realize when it is doomed, but dumb enough not to realize it's not getting anything done in the corner of the screen.
The rest is talking. If the writing were as good as UQM's, this would be fine. Instead, it's really uneven. The new aliens are a mixed bag with some very good bits, but the allies are uniformly execrable. Hardly any new race lacked something ridiculously stupid about them, but for the better ones it was less difficult to imagine a non-stupid explanation.
The best of SC3 were certain good ships they introduced -- the Pinnacle was good as-is; with minor modifications, the Doog Constructor, Exquivan Conclave, the Harika/Yorn Hunter, and Vyro-Ingo bug-sack could have been very interesting.