I know what you mean about the flickering, you think someone pops up in front of you and then a split second later they're nowhere to be seen. Are you sure you're not seeing when players spawn though? They spawn kind of half visible at first.
I've only used wiispeak once so far and it was in a private game with some wiichat people and it worked pretty well after I pulled a chair up closer to the TV. There were some feedback issues but I think that is partly an issue with someone have the wrong sensitivity settings. The other people I've played with said theirs worked fine.