I got a couple of generic batteries (Maxell) for a Canon S20 from
J&R.  Same mah & voltage rating.  I have no fear that they'll leak
or will perform poorer at higher temperatures, but I can't say that
they're identical.  They seem to weigh just a tiny bit less, and I
don't feel like cutting them open to compare them. :)  For now they
perform just as well as the original batteries with Canon's label,
but it'll take another year or two to give them their final grade.

 They probably contain 3 solder-tab AAA NiMH cells inside, and for
all I know, the Canon batteries may have bought the same cells from
the same manufacturer.  I'd be more cautious if the batteries were
Li-Ion, since that type has been known to occasionally overheat
catastrophically.  But in those cases the fault was probably due to
poorly designed charging circuits, and not to the batteries used.