I personally loved this episode. The concept of an invisible enemy may not be completely new but this episode was well written and created for a very interesting scenario.
Definately my favorite part of this episode was watching the team try to hunt down the invisible squad of Reetou. Knowing that the enemy could be standing right next to them and they didn't know was a good element of suspense for the episode. Also, I liked how the writters chose a small squad of "terrorists" Reetou instead of a huge army. Especially in our present day life, terrorists are something that people can really relate to so it added a sense of reality to the enemy. The writters and directors did a good job and creating the sense of creating a unique threat that often isn't seen with the common attacks of massive alien armies.
Another thing I liked about this episode was the boy who became known as Charlie and his connection to Jack. I really liked the sentimental aspect this gave the episode and thought that it was nice to show the softer side of Jack outside his usual hard military exterior.
Overall, I greatly enjoyed this episode. It kept me interested throughout the entire show and was definately worth watching.