We jumped on the part of the letter which bizarrely called out "muscular" cyclists for causing a danger to pedestrians. However, as many of you have noted in the comments, there were actually some fair points made about how cycle lanes and pedestrian walkways can be better segregated for everyone's safety...
Carior commented: "I was all prepared to grumble about the "muscular cyclists" too fast point but I think as cyclists we need to think about what we want. From personal experience (which includes riding along the pictured sea front) it's about as useful as a chocolate tea pot for anything other than pottering very slowly on a touristy bike ride. Pedestrians (myself included) pretty much disregard the painted lanes and you certainly can't ride at the type of speeds I expect many of us enjoy for fitness and fun along them and personally I would never use it - but then you get the "get in the cycle lane" crap from motorists.
"I don't think anyone really enjoys using those shared-use paths for anything other than pottering from A to B. That said, I struggle to believe that people would be using them riding fast on expensive bikes!"
Jetmans Dad added: "The problem with shared-use facilities is that the local council puts them in and thinks their job is done, as far as cycling infrastructure is concerned. East Riding of Yorkshire Council has done a great job of turning roadside footpaths into shared-use by just adding signs that say they are.
"They are often way too narrow even if they were dedicated cycle paths, never mind accommodating pedestrians as well, but the council is proud of how many miles of cycleway they have put in, and drivers now have free reign to abuse me for getting in their way and holding them up by not riding on the "footpath". Shared-use should not count as cycle infrastructure."
EddyBerckx summed up the general mood in the comments well: "Despite the undoubted exaggeration (though 'muscular' is better than being called 'fat') I do agree with his point as do most cycle campaigners I think - pedestrian and cycling spaces shouldn't be mixed...they only ever sort of work when there is little traffic (of both sorts) . They are bodge jobs and nothing more."