With my older DD, my MIL came for 2 weeks, my mom then for one week and my MIL again for 2 weeks.
With my second DD there was a birth accident and she did not make it. My MIL came and stayed for a month, and I really don't know how I would have survived that time without her there.
With my third DD I was put on bedrest and she was in the NICU. My MIL came during bedrest and took care of my older DD and then stayed 6 weeks after Amy was born. I needed her pretty much all that time and I consider myself a pretty competent person.
Figure out what she would be most helpful with and make sure you let her know what you need help with. Everyone loves to hold the baby but in reality you need people to go to the grocery store, do laundry, make meals, take out the trash etc.
We had our issues, but overall my MIL was great and this is why. She did all the OTHER stuff and I took care of the baby. It really is a full time job and while some people may be able to swing it with no help, unless there are issues with family that make it an untenable situation, I do think that it can be wonderful as long as proper boundaries are set.
*My* mother was really not any help because she does not drive and my father was high maintenance (he is now dead) and she was really no help. I love her, but she was no help and only added to my stress level.
I can't imagine having gone through my three deliveries without my MIL's support. Having that relationship with her has been so important to us and to my kids. No, she isn't perfect, and is sometimes really annoying. But she is there when we need her. She recently flew in for two weeks to take care of my kids as a "nanny" because DH has work travel and we have not yet gotten our act together to hire an afternoon sitter/nanny. But she was more than happy to do it. My kids adore her and I am glad they will have these memories of time with her.
Beth, mom to older DD (8/01) and younger DD (10/06) and always missing Leah (4/22 - 5/1/05)