Kind of depends upon the retirement system.
The one I'm in only has more than 10K members.
It's funded ratio is over 70%. It's been as high as almost 80%, and as low as the middle 60's.
In 2009-10 it earned 14%.
In 2010-11 it earned 24%.
For 2011-12, when the recovery stalled, it went to essentially zero. For the first 3 months of 2012-13 fiscal year it rose to over 5%, and for the 12 months ending in Sep 2012 it had earned over 18%.
I don't believe the fund can be "borrowed from", either.
Things fluctuate, but it appears that some retirement systems and funds can be managed well.
I'm just lucky that I ended up in what's seemingly a good retirement system. This wasn't known to me as a young man, nor was I really thinking about it back then when I took the job.