MacBook Pro mid 2009 reports "Service Battery" but works ok
The message that you are getting is because your battery is beginning to fail.  I can't tell you exactly when, but more than likely, when you need it most, it will no longer hold a charge.

It's actually quite easy to service the battery.  There are 10 phillips screws on the bottom.  Just remove those (pay attention to where they came out), then you will have access to the battery, memory, and hard drive.





I answered a very similar question on a 2009 MBP battery replacment.  

Have a look What&#39;s the easiest/cheapest way to replace my battery? 2009 MBP for further details.