To those of you who have yet to see this anime, please allow me to make one thing clear: Sky Girls is not a mecha fighting anime. Yes, there are mecha, and yes, there is fighting; but those things are not the primary focus.
STRENGTHS:
1. Sky Girls is a character-driven anime. If you don't find the characters interesting, then you should consider dropping it. I say this because the first half of the series is focused on character development, which can be quite excruciating if you find the characters annoying or uninteresting. I, personally, found the girls cute enough to keep watching.
2. The second half of the series focuses on furthering the plot, which is well worth seeing.
3. Fan service. Crotch shots and bath scenes where only the area above the nipples are exposed. Very minimal, but adds a bit of spice (especially to male viewers).
WEAKNESSES:
1. You might find the girls a bit annoying or stereotypical. The way they act does tend get old in the latter half of the series.
2. Filler episodes. It's sad when a 26-episode anime has fillers. I recommend fast-forwarding through them. They really don't add much to the experience.
3. It drags at times, especially in the first 12 episodes. The only thing that got me through the first half of the anime was the fact that I was strangely intrigued by the girls.
CONCLUSION:
Try it out and see if you like the girls; because that's really the only way you can enjoy this series. Otherwise, don't bother.