Simon...I agree that there is a loss in performance but especially for single handers, the convenience and ability to furl in safety seems to me to outweigh the loss of windward performance.
That said...I would not want one for blue water cruising due to the inability to deal with a furler failure or a jam and get the sail down in bad weather. This is the primary reason we opted for in boom furling. I think in mast or behend the mast furlers are fine for coastal or bay sailing.
For bluewater I would suggest something like the Doyle Stack Pack system if in boom furling is too expensive for your budget.