If you decide to leave a ranked lobby it says you will lose points, does this effect you much or should you stay once you are in.

Ideally it shouldn't reduce your trueskill if you quit before anyone has joined or after all your worms are gone but trueskill is complex, I don't know how they calculate it.

You need a PHD in rocket science to understand it completely, but the gist of it is that there is an expected outcome for each game, which is effectively based on the skillpoints / rank of the people in the game. If you play a bunch of putzers you are expected to win and a win won't improve your ranking much, but a loss will shoot you down the table. System tries to match people on their skill levels of course.

Its all documented at the URL below, although makes for heavy reading:
http://research.microsoft.com/mlp/trueskill/Details.aspx

PS - Wormetti - is that you in the top 100?

Wormetti is also my gamertag, so if my name is there then it's me. I haven't played/checked for a while.