I recently dined with a table of 5 people at Rumba, my first trip. One couple had been there previously, and was already a fan of the establishment.
The food overall was quite delicious, I highly recommend the fish tacos (though I wish the fish was a little smaller to make the taco easier to eat), and the appetizers were also quite good.
The restaurant was hopping when we were there, and it took close to 10 minutes for the waitress to make it to our table (I had to ask the hostess to get her before some of the more irate members of my party felt the need to say something), and the waitress was a little repetitious with her repartee - normally I'm not bothered by such things, but it's a small restaurant, and when you hear the same joke cracked about 20 times in 3 minutes, it gets old fast.
All in all, it's a great place for casual dining, and not like much of anything else in the Hamptons.
Be prepared for having to valet park your car, and I don't know if they take reservations, but it's probably a good idea.