I may be the only one here, but i don't watch shows just to comment about the acting, visual effects and stuff. Actually I finished this one a time ago now. However i think it deserves some more credit, as i've seen shows with poor acting and terrible visual effects that surprisingly last more than 1 season. While of course, good series (like this or better, i should say) die after 2 seasons.
I may be a simple guy but i know what i want, and it's not about the acting or the script, it's about the story! Creativity, above all. Stargate sg-1 always had terrible actors, even for the 90's, and i don't see anyone complaining. Atlantis was similar, and then stargate universe came, with great production and better cast, and guess what? Ended after season 2. If you ask me, sci-fi is going to hell these days, but at least some shows like this can still make it.
Despite all of that, as i'm not just a stupid fanboy, i will admit the second half of season 5 was horrible, so thank god they finished it there. But i can't complain about the overall story (and acting). I just can't, guys, sorry!

This is a mediocre show with shallow characters and an even weaker plot. There are multiple reasons contributing to make this a mediocre show.
1. Nationalism
AMERICA!!!
Why do you have to shove nationalism down our throats when the antagonists are the alien overlords. There is no government or countries remaining after the attack.
2. Characters
Characters are one dimensional. The MC is portrayed as an intellectual specialising in history. When the show starts out they tried to show his intelligence and rational thinking. But as the show progresses he becomes dumb. Plans are horrible. He is impatient and takes unnecessary risks. Other characters are even worse. Most of the characters don't act like they are facing an alien invasion. And the characters have zero development.
3. Plot armor
There is some obvious plot armor to all the major characters. Even with 99% fatality rates, they survive multiple times. Shoddy writing makes the plot armor so obvious. Even with the stupidest plan they face no major deaths and with some lucks they succeed.
4. Plot
Plot is all over the place. The story is about an alien invasion of earth that focuses on US of A. The MC, a history professor is the main attacking force of the whole world. Don't worry he has his 3 sons as his back up. The direction tries to make the MC as masculine as possible, that it comes as a bit odd. The story is messy and feels like a 30 minute episode stretched to 5 seasons.

It starts really well. The first couple of seasons is really enjoyable, but then they sadly start to add more and more things and it turns into a really chaotic story without a clear direction.