Should you buy Diplomacy? I think that depends. Do you have Entrenchment, in which case it will only be $10? If you're comfortable playing it in single player then Diplomacy (on top of Entrenchment) is probably worthwhile for that alone. It doesn't really add all that much for online multiplayer, so the reason to purchase it for multiplayer is to be able to play with all of the other people who have Diplomacy. Most of the humans v. humans action, the 3v3s, 4v4s, and 5v5s is on Diplomacy. However, do be aware that this game has a small online player base and that you might have to wait patiently to find games to play and for games to fill. If you're in, say, Australia that could be a problem. It seems like most of the action is for North American prime time. Also, the other issue is whether or not people will be playing the game 2 months from now. I say go for it if you already have Entrenchment and really like Sins.
As for balance, the Marza is still the same but I think the length of the missile barrage was increased and the damage slightly decreased. Illuminators are still powerful but they received a significant nerf and it was even discovered that they were doing unintended extra damage (the "Illuminator Bug", which was squashed). The good news is that a major cause of minidumps and desyncs was identified and eliminated.
Right now the most common theme is that you will see early game dual Skinatra rushes. The Scramble Bombers ability on the Skinatra received a buff when Diplomacy was released, making it overpowered early-game. It can be countered if you see it coming.
