Well, I don't know if I would go so far as to say SP2 has "no impact" on
programs or performance. Simply because of the vast number of lines of code
that are added, altered and otherwise fooled around with by SP2 it can, and
does, have an effect on every system it is installed on. That effect can
vary from negligible to profound depending on the environment it is
installed in. I too have seen it installed on dozens of machines with no
perceptible problems. I have also seen it installed on identical machines,
with identical software and hardware and seen it toast one machine while the
other hums happily onward.
I have seen it installed on a machine with anti-virus software up and
running at the time of installation, supposedly a big no-no, and it
installed perfectly. Service Pack 2 is a major upgrade of the operating
system. It should be handled as such, and installed only when absolutely
necessary.
I have not installed SP2 on my personal machine as it provides no benefit to
me to do so. I install all other Critical Updates and I protect my system
with a firewall, anti-virus and spyware software and some good old common
sense.