Yes, the online is still to my surprise relatively active, when I make a custom ranked match opponents show up quickly, the ranking system they use is maybe one of the best i've seen in any game period. Im currently the number two F rank out of 86 other people who are a F. The Ranking system goes from A-J (10 ranks A, B, C, D, E, F, G, H, I, J), and the top 30 people with an A are designated as "Rankers." Its pretty sweet the top 30 rankers get a picture of their AC design, their emblem, and a brief message profile kinda thing with their stats. Most people online though are A's and B's so theres a big learning curve to get over and they'll kill you a lot at first.
As far as Gameflying this game goes I wouldn't recommend it unless you just want about 500-600 points that the story and the few easy online achievements can net you. Also it took me about 45 hours to beat all the missions on normal and hard with SS rank so it can be a very time consuming game. If your really into the Armored Core series however, this one is imo the best armored core since Armored Core 2 on PS2 and definitley worth just an outright purchase. Theres to much work and labors of love that go into an armored core game to just rent it. You can get this game used now for like 22 bucks