I have never been able to get the World of Minecraft client to actually work, however I do use it to play.
After playing Minecraft (with hacks using the World of Minecraft Client) at my Aunt's house on her laptop for a couple of days, i got home and decided to play. Thinking the hacks would not work i button-mashed zqeqeqeqeqShiftwwwww and i was surprised to find it was actually working! So i played as usual (but with hacks) and i started to get a bit laggy after doing a cuboid cmd so i decided to refresh. I clicked Reload and when i got back into the game i realised the hacks were no more :( they had randomly stopped working.
I don't know why. I don't even know how they started working. I always used to think that maybe i had placed the Minecraft.jar in the wrong folder but obviously i had not as it was working for about half an hour.

I do get a random error-like message when i play sometimes though. I can't remember exactly what it says at the start but it's something about other player's skins? for example:
(blahblah)minecraft.net/skin/nameofplayer.png or something like that. I will get the (blahblah) bit and post it here if it pops up again to show you guys. But does anyone have any advice they can give me? or answers as to why it randomly started working and then all of a sudden stopped working?  :)

EDIT: I'm using windows 7 and i also get an error that says:
WARNING: found unknown windows version: windows 7 - but  i think everyone that uses windows 7 gets that? imirite?