I just watched the final episode, and I must say that I thoroughly enjoyed myself.
The first half, with the volleyball tournament was quite epic. I loved seeing Eiko and Ika team up together, and their match with Goro was action packed, and made me want more action scenes like that throughout the series. It was also funny to see Ika trying to grab the shrimp in 3d. I probably would have done the same thing.
The rest of the episode was rather touching, with Ika having lost her squid abilities, and then gaining them back after having to go save Eiko. I must say that I thought Ika did look cute with short hair, but I really do prefer her the way that she is normally.
I'm slightly sad to see Shinryaku! Ika Musume end so suddenly, because it was a funny, slice of life series, and we can always look forward to the extras on the BD releases, and the hope of a second season.