Pros and Cons to living with your mother in law?

My mother in law has been in the hospital 4 times the past 2 months and has a long list of ailments. My wife and I have to help her with several things and care of her house and animals when she's in the hospital. We have a house, kids, jobs, and pets of our own, but not enough time because we're constantly helping her.

We are considering buying a bigger house and inviting her to move in with us and charge her rent of $800-1000 per month.

We could use her (when healthy) to get our kids off the bus when they're old enough, she would cook some meals, be home to let our dog out while we work all day, and she would help us financially. However, our family dynamic would change and we don't want it to affect our marriage. I'd like to hear from anyone that has lived with an in law. What were the best parts about it and the worst parts about it?