This stand-alone expansion to one of the most epic RTS lately is surely worth the low FPS on most machines.
The fact that you could play epic or even gargantuan battles on battlefields greater than any seen before on the computer made the game superb. Only downside were it's system requirements. I can imagine that a lot of people dislike the game because of it, but when you downgrade the graphics enough, you can play the game quite well.
The same goes for the expansion. I played the Beta using Sandbox mode with some cheats to play battles against my own. The graphics surely did improve at the cost of your system, but once again, you can take this FPS-Loss for granted. You get so much in return for it. The fourth, new faction, is really a great addition to the other three. At this moment, I haven't played the campaign yet, or tested the new AI, but despite some bugs in the beta, which I think GPG will fix within a short period of time if they haven't already, I think this game is a very good successor indeed.