Falling Skies is fine in terms of acting, script, production etc (although I thought that the CGI versions of the aliens looked unconvincing, but that's a fault shared by many series from this period). It's just that the programme offers nothing new: it looks too much like a re-hash of the post-apocalyptic scenes from the Terminator films, combined with some un-used footage from Spielberg's (dreadful) War of the Worlds movie. Which brings up what, for me, is the biggest negative aspect: executive producer Spielberg has seemingly used Falling Skies as yet another opportunity to play out scenes of father-son relationship issues. I thought that he had moved on from doing that but evidently not: in this series the lead character, played well by Noah Wyle, has been given three male children in order to get the point across - one of whom looks too old to convincingly be the son of Wyle's character. As in Spielberg's WOTW movie, there's too much of an obvious attempt to glue a family drama onto a sci-fi backdrop and we've simply seen it all before by now.
I'm a lifelong sci-fi fan, and will usually give a series a good chance before giving up on it, but in this case I won't be staying to the end as I feel like I've seen it done all before, only better.