Personally, I use Chaos Spawn, mixed with some random Tyranid bits. That seemed to work pretty well. Virtually anything's better than the ludicrously expensive/ugly official models.
Daemonette/Spider Rider hybrids make much better Seekers of Slaanesh then they do fiends, in my opinion (that's actually what I use for them).
The first casualty of war is neither innocence nor truth.
The first casualty of war is always subtlety.