HU&W nailed it..essentially it's a use it or lose it kind of thing.
For me, I'm getting an OHA of €1196. But our rent is €1200. Hell I'll pay the extra €4 a month for a nice free standing house with a yard, in K-Town.
Monthly utility allowance is where you make your money, don't waste energy and you'll make bank. Now, if you are thinking, "I'll just have the land lord include utilities in the rent and bank the €615/461!" be forewarned that housing is on top of this game (I was warned like ten times in my initial meeting). They'll cut off your utilities allowance apparently.
Don' forget all the other allowances and such for being overseas, moving, ect, ect. If you are smart, you can come out pretty well.
If you don't go through AHRN, expect housing to be an epic pain in the ass about it. Heaven forbid you make them do some work and go inspect another house. Expect the 20 questions game about why you didn't use AHRN. Expect to get shit about having to use a realty agency to find a place, never mind the fact that AHRN listing suck currently. There are no exceptions to the rule, no waivers, no helping a dude and his wife out.
What is really retarded about the whole thing is the way the calculate how much a place is 'worth' and thus how much of your cap they will give you. It's 90% based off square meters of a place. For example, a 100 sq meter house is no more valuable than a 100 sq meter apartment. They do take into account garages, yards, ect, but it's only the difference of a few cent's per sq meter. It's absurd and a huge ass pain as I'm finding out.
So my suggestion as far as looking for a place, in order of least ass pain:
1. Find someone who's moving out, since their place will already be inspected, ect.
2. Go through AHRN if you can, pickings can be slim, and it's very cutthroat come summertime, but less pain with housing.
3. Use a realty agency, aka immobilien, yes you have to pay a user fee, but they do find nice places quick. See my warning on housing's opinion of this route.