Yes, they share beakers, tech and los with you. However, they also are considered as being one of your cities for the purpose of building wonders. And you don't appear to gain the bonus from wonders they build, so you'll find yourself fighting to start a wonder before they do. They also share building of space parts, which is very frustrating if you have a city producing 100 shields a turn, and they start building a part in a city producing 5 shields a turn.
For the most part though, it's nice having an ally with you. Any time they have a lux/resource that you don't have, they'll immediately pop up a trade window and offer it to you for free. You should probably do the same to them, give them any of your spare resources, to make them stronger.