I've actually quite enjoyed Windsor the few times that I have used it. Like dmtaylor, I have only gotten very low 60's attenuation (59% once), but the beer never comes across as sweet. A bit full-bodied, maybe. "Smooth" might be the best way I could describe it. Of course, I've only ever used it in a Porter so I'm assuming whatever esters it is kicking is getting lost behind the grains. Maybe I wouldn't care so much for it in a Bitter or a Mild.
As far as London, I was hoping to get somewhere closer to 70%, but from Dave's earlier post and the current visual inactivity from my fermenter after 2 days post pitch, I'm thinking probably not. I mashed at 150, but maybe I should have been at 148°. Would it even make a difference? I'm beginning to think that for a Porter, there might not be much difference between London and Windsor. I'll know for sure in a few weeks.