Two days ago I would have said rent. Now I just voted "buy." Today, I joined a 20-man clan (the max you can have), where our team level is at 151 (and rising quickly). Let me break it down a bit.
The game is always online (I wouldn't know what happens if you have no internet connection). Your clan is always posted on the right side of the screen. In my clan, there are always at least 4-6 people on; at one point, we had 13 people on.
Now, when you start the game up, you are at this big world map. All your clan members who are not on a mission are able to talk (basically a huge party chat). This might seem like a problem (13 people trying to talk at the same time), but it's actually fucking awesome and quite coherent. From this world map, there are many options. You can run story missions alone or with a clan-mate or mercenary. You can run "order" missions, which are small side missions (which get a lot harder over time) that you can run with the same options (alone or with someone).
When you select a mission, your clan is notified with a "Rendevous with 00" - 00 being your internal clan designation. Moreover, you can run large territory missions with up to 5 players: 4 combatants and 1 on overwatch, who can spot enemies for the people playing. Finally, you can battle other clans for ownership over a piece of land in a territory; once you own enough of those, you can own the entire territory. So basically, it's like taking over cities in a state to control the entire state. Everybody online sees this. You can either choose to attack or defend these areas.
Every mission you complete adds to your team score. So essentially, you are always improving. Now, that's just the multiplayer. Wait until you try customizing your mech. It's fucking sweet as well. So many options, so much detail. It's insane. Fuck renting this game, you'll keep it for too long IMO.
End game review.... I could have gone for another 3 paragraphs about customization.