After flying BMI serveral times, to avoid a half day layover in FRA, here's my impression. BMI have very nice well appointed and very British appointments. There cabin crew if very attentive and easy on the eyes. For the relative short flight to DUB, is my not be worth the hassle. If you fly CO, you will be walking FOREVER to catch a loser cruiser (bus) to T1. If you fly on UA, you are still walking for awhile to recheck through security (very basic) to walk to the other end of T1 to their lounge and gates. The shopping is OK, they have a well stocked HMV and you will be a small quick flight.

Biggest issue is the, though they are currently owned by Lufthansa, you don't get the same benifits, ie. no 500 mile min and NO bonus miles. So unless you need to go to LHR, I would go direct.