My two fastest halfs were achieved by running a negative split. They were also the two that felt the best physically and mentally.
Conversely, whenever I've run a positive split I have felt like I'm hanging on at the end and just about fall over the finishing line.
The tricky thing about running a negative split is judging the relative speeds that you want to run in each half. I work out the average pace that I want to run the entire race and then add 5 seconds per km for the first half and subsequently subtract 5 seconds per km for the second half. That seems to work for me, you could be different.