I LOVE the Jotnar. They are a great race and very unique.
Now, I have to say it: they are overpowered. A little in strenght, a lot in production. Everybody is talking here about immortal/deity difficulties, which are supposed to be hard. Take it down to prince/monarch and you'll see...
They don't need neither farms nor grasslands. At all. Windmills and watermills provide all the food they need to get to size 8. Only one farm is needed in cities without rivers (so a farm irrigated through construction, that is), never more than two (specially once you get sanitation).
Their commerce output is HUGE! Two of their leaders are financial, which basically affect all their windmills and watermills. Was someone ever short on money? Was I the only one who experienced an amazing research rate?
As they don't need that much food to make their cities grow to size 8, who wants grasslands? Take Ergrass and make everything plains for the extra hammer. So their hammer output will always be pretty good as well.
Then, at the same time, this is extremely easy because you can use every citizen to build improvements. In the practice, this means that any newly found city will see its surroundings improved in a few turns, and it will get to size 8 extremely soon. I never bothered to build a worker. Egrass and the citizens is all you need. And I mean like... REALLY fast. A city is founded and next turn it has 3 citizens working its tiles. I was discouraged from upgrading citizens as long as I didn't need to, so as to use them as workers.
So, to slow them down, citizens shouldn't be able to build improvements. Upgrading them to workers, as someone suggested, would make them more balanced. Then, the +1 extra commerce for their windmills and watermills should be taken out. Financial leaders will already take advantage of these improvements later (it's at engineering, I think, when they get +1 commerce).
Playing as them (on prince, I think), I got on top of the score pretty soon; then I killed the Mazatl with a few giants in a matter of some 20 turns. I only played until turn 250 or so, but I already owned the game. So I don't know how they do at later stages.
A question. I thought you could only have 1 mouth of the divine. Has that been changed? I could upgrade several citizens to that.
On the positive side, the citizen spawn rate seems fine to me. You don't get many, but they are extremely powerful in comparison to what other civs have at the same stage of development. Also, I see the Jotnar as an easy and very interesting race, quite good for newbies. Well, really, the Jotnar are amazing, but maybe too amazing...