We were thinking about doing it, but things changed. We wanted to use my brother's for her, but he was killed this year and was my only sibling. We still wanted to use her brother's for me but since it couldn't be balanced with DNA from my family, we decided against it. The option of using my cousin's is there, but I think we've decided to go with anonymous donor sperm. My DW has issues with a known donor being "known" by the child, as in "that's my dad".
I also agree with the PP- I think it's ideal to have the child be biologically related to both partners. It's VERY important to my DW, so we've decided for me to carry her egg and then later, me carry my own. That way we each have a biological child and I get to give birth twice, whereas she's not really into that. We'll see if our plan manifests in a couple of years.