One of the interest groups I belong to switched from Yahoo Groups to io Groups about 1 year ago in frustration at the continual problems with Yahoo Groups. The transition was seamless and very pain-free. It is a free group system just like Yahoo, and we have all been very pleased with it so far.
Groups io was created to replace Yahoo Groups by the same guy who wrote the original Groups product that Yahoo bought.
I’m an unofficial Yahoo Community activist, and Groups io is ‘our’ lifeboat-of-choice should the stricken ship finally sink.