I like them only if they don't break the pronunciation or uglify the word.
Some I don't get at all. Like shh for double sh... That is not double sh, that is inexplicable. Very often doubling u won't work, as double "u" will be read as "you", not "u" as in "umbrella". Or, you shouldn't be doubling letter after vowel, if turns it to short vowel from long. For example, "skater" to "skatter" should not work.
You have to be able to communicate the brand with saying "wordd with double d". For example, regged sliing. I can say "go to sliing with double i".
I am amazed how many of those ugly ones are accepted at BB...