The concept behind the series is not bad. It borrows a bit from an older series called Sky Girls. There are differences indeed. Sky Girls does not have mechas behaving more like power suits that fit into a compact form they wear at all times. They also are more like personal jets than suits like this. When you look at them both, Infinite Stratos has an edge on design. For that reason alone, IS is worth a rental.
As a harem anime, IS fits in on par. Characters behave in traditional patterns and there are the common characters rehashed. You can somewhat predict how the story will go from that. If you are a fan of these sorts of shows, you will find some decent scenes here and there. It isn't a bad show and it is nice that there really isn't one completely clear winner girl throughout it. You know which girl is going to get more screen time but each of the girls make an impact on the guy, some more than others. The potential for a sequel is quite strong.
Had this series been released years ago, it might have been cleared for a full 24 episode season. That is perhaps one of it's biggest weaknesses. It is just a single season. Some of the character development with the other girls gets cut and the series tends to rely more on, the clueless guy with girls fighting over him, concept. That alone causes it to drop a star.
Harem animes have tended to slip more into this format in the past years. Something which is a bit of a shame. Love Hina is the textbook example of a good harem anime and it still is. Too often writers use the girls fighting over the numbskull to insert fanservice. Generic story + generic characters...just insert dumb guy...add girl fighting...blend!
The SciFi aspects of this series and the mecha designs alone are what brings IS out of the generic toilet. It is a bit generous to give it 4 stars but I can't quite give it 3 stars. So 3.5 rating on this would be best. At the very least, give it a rental if you have a renting service. It is worth at least that much.