I like Northern Brewer and especially Willamette for bittering. I don't feel they're out of the norm or impart any "specialty" flavors. The Will's are low-AA% so you'll need more of them to get your point across.
Clusters work really well with more complex malt profiles. If you try to do a base-grain only brew, the Clusters tend to make it taste a little like a freshly mown lawn. I brewed a six-malt amber with them though as the 60m hops, and with Will and Cascade as the late additions, it was flat-out awesome.