We use primary nursing and really love it. It's entirely voluntary, and "dropping" a baby once it's not sick anymore is really really looked down upon, so it doesn't happen very often. We try to make sure each baby has a primary nurse (any shift), and then we can also sign up to be associate nurses to take care of the baby when the primary isn't there. Some babies have up to 8 associates!
We have a big board where all the babies are listed, with the primary nurse's name next to it and then the associates after that. The charge nurse takes the assignment sheets for the next day to the board and pencils in primary and associate babies next to the nurses' names. Of course they try to assign all the primary nurses to their babies first. Then they pencil in the associates, and these are given second priority. If there are multiple associates on for a single baby, the one whose name is further up the list (signed up earlier) gets first dibs. If a baby's primary won't be in until evening or night shift, the baby is assigned to someone who will be leaving when the primary comes on. It works out really well, considering that we do 4, 8, 12, and 16 hour shifts.
Primary nursing works really well for us, as we enjoy continuity of care and feel that it really does help in terms of picking up little changes from day to day. The parents really love it too, especially because we do associate nursing as well. It seems that some babies almost always have the same nurses, day after day, and the parents have said that it makes them feel so much more comfortable. Of course, there is a downside here - it makes parents really wary of "strangers" sometimes. You have to explain that it's an ICU and that they won't always get "their" nurses but that everyone is very qualified even if they don't "know" their child.
For me, it makes me excited to go to work to see "my" babies and how they're doing. Because we've bonded with these babies and their families, it makes such a lasting impact on them. Parents say it intensifies their rollercoaster ride in the NICU to have someone familiar to celebrate with or to greive with. They know that we spend more time with their children than anyone.