I second everything Casey C. said!
I am blessed to have a wonderful MIL, who takes care of our kids while we work. In the thirteen years I have known her, we have never once had a fight or even a disagreement. I think this is mostly because she is a truly awesome woman.
Here are some of the things that make her great:
1. She never offers her opinion. If I ask, she is happy to share. I think this is the number one key to a good MIL relationship. Whenever I see people complaining about their MILs (or even Mothers!) it is because of this.
2. She always supports what we do with the kids. She didn't BF or co-sleep, but she supported us 100% in our efforts.
3. She never "overstays her welcome". We'd be fine if she LIVED with us, but as soon as we come home from work, she heads out. If we invite her to come with us somewhere on the weekend she demures. We took her on our vacation, but we had to beg her to go with us. She gives us plenty of space. (She might also get sick of us...)
4. My husband is an only child. With my MIL, I don't think another sibling would be an issue, but my husband is the only child, so that makes us the only game in town. I could see that working against the relationship (if the MIL was too attached to the only child) but for us it just means we don't have to share her! And there's never any "sibling" drama. (At least not on his side... my side, that's another story.)
5. She is a genuinely kind, trustworthy, warm and funny person.
6. She loves are children unconditionally... sometimes I think even more than we do!
7. She's not my Mother! And she doesn't try to be. I have a complicated relationship with my own Mom, but with her we don't have that baggage. Its more than a friendship, because she does so much for me.
Finally, she respects us and we respect her. I always say she's setting the bar sooooo high for me with my kids... I'll have to do her proud someday when I have my own grands.