I am trying to weigh up the pros and cons. The biggest pro is that the DUB-CLT flight clears immigration at DUB.
The Cons:
Its US Airways
Its not United

I heard horror stories about immigration at both IAD and IAH. This will be probably my last miles and cash redemption, and wanted the UA/CO product. US is a bit hit and miss in a 757 out of DUB.