Hello and welcome to the forums! 
Any game that is not chess or something similar will always have more or less fiercy balancing discussions. You should not let yourself stop by them.
Rebellion offers many "under the hood" features that I really dont want to give up anymore. It is certainly worth buying. 
There is also the fact of Stardocks awesome support for this game.... many of the balancing problems you have read about are fixed by now. And their support is continuing. 
The remaining issues mostly concern competive multiplayer and as such should not stop you from enjoying all the new cool features!
Against the AI and on "casual multiplayer" the balancing is pretty good. You should have little issues.
The AI received some major improvements with Rebellion.... yes... it still isnt the brightest bulb in the fridge but it is an considerable higher threat overall than in Diplomacy. Sometimes it is downright brilliantly....as I learned the hard way a while ago.
The Titans alone are also of considerable worth.... they add lots of new strategies and options to the game, besides it looks awesome to have one.
Which brings up another topic.... graphic enhancements... Sins Rebellion looks considerably better than Diplomacy... especially the effects have been made a lot better. Add a nice graphic mod and Sins looks better than ever.
Other things that come to my mind.... there is more, this is just a small list:
- Advent superweapon now actually useful
- A large ammount of new maps.... many awesome ones too
- Fair maps... no longer can your housemate claim it was a bad starting position for him when you happily nuke his capital into oblivion
- New capital ships.....
TEC - The Corsev..... Boarding party in stand by..... that frigate/cruiser is mine now. It has a few really nasty abilities that supplement the boarding... like placing high explosives on boarded ships and then blowing them up right in the middle of the enemy formation... ouch..... and it is a tank.... your Kol is long since an expanding gas cloud where this baby still dishes out pain in all directions while just refusing to die.
Advent - Discord Battleship... very nasty.... ravage frigate fleets or make the enemies armor much thinner.... or what about putting enemy ships - including Titans - into a state of lethargie? Ohh... and dont even speak about Level 6... "Revenge from beyond" You just destroyed 20 of my ships.... How dare you! Now, those destroyed ships take revenge from beyond.... by doing massive damage to the enemy fleet.
Vasari - Rankulas Battleship.... this new support capital ship is still searching a role in the Vasaris battle plan. They may be changes to it in the future.
All summarized... I would recommend buying it.
Any further questions? Ask away!
