With The Event and V already on television, the question of whether or not audiences want another alien invasion TV series is definitely a valid one. Without having seen a full episode of Falling Skies yet, my viewpoint could very well change, but as it looks now, I think Falling Skies will end up being the only true "alien invasion" television show out of the three.
Let's look at The Event first. In the beginning, the aliens were actually portrayed as the victims. Most of them were locked up in prison and just about all of them were friendly and had a code of "do no harm" to us humans. It's only until recently have the aliens turned into potential invaders. Also, the show also has very little in the way of alien technology. Most of the tension and drama come from the various politics at play. I'm not saying there needs to be a bunch of alien spaceships or anything, but you could ditch the notion that there are aliens at all in The Event and it'd still play out nearly identical to how it's going right now. So far there hasn't really been anything relating to an "alien invasion" storyline. The last few episodes started ramping up the possibility of an invasion, but the invasion seems more like a normal human versus human conflict rather than one coming from outer space.
While The Event's relation to an alien invasion show is quite thin, V is another story. On the surface, V is nothing but an alien invasion show. After a couple seasons, however, we still have humans thinking the aliens are benevolent. It's pretty clear that the "invasion" aspect of the show is more of a carrot at the end of the stick that we keep waiting to happen. This last season ended with what seemed to be a first strike by the aliens and the reveal that the humans have prepared their own defense force... so perhaps the show will turn more towards and actual invasion story. Until then, I full expect to see the story continue with the aliens hiding their true intentions to the masses and doing a more sneaky invasion rather than a full on war.
This brings us to Falling Skies. From what we can tell from the previews and the web comic, not only is the show very much an alien invasion story, but it's pretty clear we've already been invaded. Basically, the show seems to be what it would look like after the Indepenence Day movie if the aliens had won. The aliens beat us and we're struggling to survive. They've got bigger guns and better tech, and we've got... well... I'm sure we've got something. This is why I'm so excited for the show, because it really is about us trying to survive against an alien invasion. Sure, there have been plenty of movies that have covered the topic, but this looks like it may be the most promising serialized version of the genre we've had yet.
So, with three different alien shows I definitely see room for one more - especially when this one seems to be taking place right in the midst of an actual invasion rather than teasing the possibility for season upon season.