In my opinion, they're buddies that were driven apart by conflict. Here's the best way I can describe it. I played on a pretty good football team in college. We should have finished the season undefeated, but at one point found ourselves losing in the second half. All of us on the team were pretty good buddies at this point, as you normally are when you spend that much time together. However, on the sideline of this game we just couldn't stop pointing fingers and complaining about what was happening. This eventually led to our loss in that game.
This is how I see Brutus and Cassius. They are obviously bonded strongly by friendship as we can see during their conversations in act one and their farewells at the end. But when things started to look a little bleak, they really struggled with their own friendship. They did manage to patch it up just in time to kill themselves, but that rift in act four had to be weighing on both of their minds.
I think even more than friendship, they probably respected each other's ideals. Cassius certainly used this against Brutus to get his own means, but isn't that what friends do from time to time? Granted this is a little worse than what we would consider day-to-day, but the principle is still the same.