I personally would prefer people to say that they dont like a map, rather than sitting around waiting for comments, as without comments the map wont go anywhere.
As for giving feedback, if i have a problem with a map i always try to give a suggestion on how to improve something if im going to comment, otherwise the map maker cannot actually improve anything and so my comments have been pretty much pointless. And always try and be slightly encouraging.
Timing: i reckon it is never to late to raise a concern, providing you are reasonable about it, and once again give suggestions on how to improve, rocking up in a beta thread and saying "this maps sucks, i cant read it, and it looks horrible" is not really going to help anything, apart from maybe the map makers foe list grow.
But generally i dont like posting in a topic if i have not read most of it, as it helps no-one to drag things up things that were discussed and decided on on page 2, although i would hope that if i did the map maker would be mature enough to just tell me it had been discussed point it out and then move on...
Oh and always include at least 1 smiley to give the map maker an idea of the mood you are in when posting so they know how seriously to take your post.
![Wink ;)](./images/smilies/icon_e_wink.gif)