Renting it is the perfect amount of time to get what you need out of it
First, the Story Mode. It tells an excellent story, I'll give it that. I was completely enveloped in the cutscenes and dialogue and the history, initially. But then, I realized that the game is making me play with different characters every level and half the time they aren't even enjoyable ones. I dont know about you, (and I'm using Shu's story mode as my example) but I don't really like playing as Liu Bei or Zhuge Liang. I'm a Ma Chao guy, but he doesn't have a single level. Almost every level you seem to play as a new character and you see his perspective on the battle, and it's cool sometimes, like when you play as Zhao Yun on "Chang Ban" and save Liu Bei's son, and once you reach the bridge, your character switches to Zhang Fei and you hold off the people on the bridge as the peasants escape. But you only play as a total of probably 8 characters over 20 levels! I mean, you play as Zhuge Liang, Liu Bei, and Pang Tong, but you don't get to be Ma Chao or Wei Yan or Guan Ping?? Needless to say, after completing Shu's story, I was missing Musou mode.
Then, there's Conquest Mode. It seems cool at first, but then you actually play it and it's a whole different story. There's a total of (I think) 6 types of battles, and they're all easy and quick, oh, and boring too. You can bond with officers that you fight along side of to the point where they will become your sworn allies, which is cool, except all they do then is follow you around in battles and protect you. I miss the days where you could tell your officers what to do (like protect bases, etc), but no, not anymore, and your "sworn allies" just follow you around, essentially becoming useless. Conquest Mode is a bad replacement of Free Mode, especially since the battles are usually annoying and customized in random ways (with random generals and stupid missions) that always seem to make it less fun to play.
Overall, you can just rent the game, play it for 2 hours, think it's good, play for 2 more hours, realize it's horrible flaws, and then return it. The lack of Musou Mode and Free Mode is a huge mistake. The only thing the game got right is a more enveloping story and the new two-weapon system.
One last thing, In my opinion, one thing could have upped this game's score atleast a point, and that would have been if (after you complete a kingdom's story) they would let you replay the levels with whatever character you wanted, not the same character that you played with before. I would have loved to fight through the story mode again if I could do it with one guy that I enjoyed being. Just food for thought..