First Wave was bascially a modernized reworking of the old Quinn Martin series about aliens appearing in human form: The Invaders, although creator / executive Producer Chris Brancato says he never saw that old show.
It ended in an over the top manner with Cade and his friends overcoming the Gua -- driving what's left of them to leave Earth.
First Wave was an entertaining show, but 3 years was more than enough. The addition of Traci Lords in the last season turned out to be a plus, and the writers got to do a lot with her character. Ironically 3 of the 4 leads were all good-looking people. The oddball in the bunch was conspiracy theorist Eddie.
This series was never a major hit -- at least not here in the USA. In its native Canada, First Wave was well received and the Canadians saw episodes on their Space Channel before our Sci-Fi Channel finally aired them months later. I was fortunate enough to deal with a woman in Canada who taped some of the later shows for me so I could watch them early.
Low ratings are ultimately what killed it, which is why Brancato and his writers went the happy ending route.
I remember I used to watch it when it was first on scifi, debuting alongside Farscape. I don't think I saw any of the last two seasons though. Might be nice to catch up.