I like this civ. I was so pumped for new civs when we started to get the more unique ones (Spain, Kongo, India) and every civ seemed to be very special. Rome and Greece were a letdown in that regard, and I'm very happy Sumeria offers a much more different gameplay again.
The UU and the UA work together very nice in the beginning. I will probably build a lot of the war carts and chase barbarian camps. This might reduce the need for scouts and help with city states, too.
UI is of course great (despite the look) and I was hoping for some extra science with Sumeria.
I like the 3-part leader ability very much. It requires you to have one or two good friends in every game to be most useful. I might just join them in any war possible, even if I just hope move some units along. I hope it also works with city states, since defending them from other civs might get a lot of xp. It's nice how everything with this civ works for advancement, too. Barbarian goody huts/Ziggurat/shared pillaging all give you a great advantage, and we would probably want the AI to pillage Campus first ;-). I also believe in Firaxis, that they make the AI consider Gilgamesh's ULA both for AI civs and for players and are more likely to join/start wars.
I am a bit prejudiced of course, since I am such a big fan of early history. I can see where they got the bonuses from (and I also think Gilgamesh's fits nicely), but I would have hoped for a housing bonus. Since Kongo seems to be *the* tall civ now, I already got rid of that idea. Would have preferred Sumeria though.
What I don't like is Gilgamesh's look. It scream a whole lot of things before I think of Sumerian. Sumerian art never depicts kings or gods as that physically powerful, they usually look like normal human beings. And the clothing and head gear also seems to be not Sumerian at all. There are not many Sumerian depictions of Gilgamesh (6 that I'm aware of) and none looks remotely like that (also: none is contemporary). Instead, it reminds of Assyrian depictions of Gods, Kings and Heroes. I know we had the accuracy discussion already with every leader. I haven't complained much so far, but this is like having Vercingetorix being the model for Louis XIV.