I have flown Aer Lingus to Seattle a few times from UK. Outbound is fine. Clearance in Dublin was easy and as has been said its better to get that out the way whilst awake. In business class the Aer Lingus offering was also significantly cheaper when I flew and the service was great.
Its a pain on the way back but at least currently you dont get any immigration check back in the UK arriving from Ireland. I would recommend it particularly if you are more than a few hours from heathrow and can use a regional. As an aside I assume the simple arrival from Ireland into UK will end shortly?