How "safe" is my laptop running on Service Battery
I replaced mine (MBP 2009) as soon as possible in the knowledge that Apple only stock replacement batteries for 5 years (source: Apple Genius told me). Mine was 4 years, 11 Months...

Aside from the 'service battery' warning I was seeing wildly fluctuating capacity and a tendency to power down hard at ~20% - so aside from the physical risk you may also risk data loss &amp; file system corruption.