Sorry... long response!
My parents live with us (DH, DS, and I). My sister might be moving in as well. We own our house with my parents and have lived here for 2.5 years. parents moved in in April. We are unemployed and will soon be moving. My parents moved in so that DH and I wouldn't have to draw against our investments to make ends meet while I look for work. They also wanted to spend some quality time with DS before we moved. My sister might be moving in becuase she can't make ends meet. Our living situation will over lap for about a month until we move out in July or August.
So far, it really could be worse! We moved out of the master bedroom so my parents could have some extra space. DS has his own room and DH and I live in our former guest room/office share a bathroom with DS.
We do all of our meals together and we give my parents money toward food, since they like to do the shopping and we could care less. We do the same with utilities.
The rule is that if something is messy and you're close by, then you clean it up. We take turns making meals- my mom tends to cook dinner 3 days a week and I do 3 days a week. If my mom makes dinner, then me and DH clean up. If I make dinner, then my dad and DH clean up (yeah, DH does a lot of cleaning!

). This is pretty much an unspoken arrangement. DH and I take care of cleaning the common areas (living room, dining room, etc.) since DS makes most of the mess in there. My parents help out with gardening. They know that my room and DS's room are off limits- we take care of the cleaning in there.
Some of the things that we've had to work on: my parents felt like the TV was off limits and they couldn't watch their shows. So we let my dad take over the payments on the cable so they felt "ownership". Weird, I know, but now they feel more entitled to watching TV. DH and I usually retreat into our room at night to do homework or play board games so that we all don't feel like we're constantly on top of each other.
And I had to make it CRYSTAL CLEAR that DH and I are the parents of DS and my parents are not to discipline DS unless he was in a life-or-death situation. The first couple of weeks, DS was receiving a lot of correction from all of the adults, and my parents were sending DS mixed messages. Once we made it clear that just becuase they lived here, it didn't mean that they had to be the parents of DS, things relaxed a lot. We also had to explain that since they now live here, they can no longer shower DS with sweets and junk food. They were chagrined, but now they bring home strawberries instead of chocolate.
We also realize that it's exhausting for DS to be around my parents all the time, so we've been trying to take DS out a lot so my parents can relax.
It's going pretty okay so far. At least, it could be worse. I left home when I was 19 and never looked back, so it's TOUGH to be 33 and living with mom and dad. My mom is dealing with the fact that I'm an adult and just becuase I come down with a cold or indigestion, she doesn't need to call the doctor.

This has been an adjustment for everyone, but it's only temporary!