The battery was charging completely but only holding a charge for about an hour on my mid-2009 MacBook Pro. At the battery icon pull-down menu there was an alert to "Service Battery." I researched the battery on iFixit.com and ordered an "A Stock" unit. I followed the easy on-screen guides and within an hour of receiving the battery my MacBook Pro was operating normally. The replacement battery works great!
Easy. Took less than 15 minutes to do.
I would do as I did, research the battery, review the installation guides and make sure you have the correct tools to do the job.
If you do the above, you are virtually guaranteed a positive outcome. Beware, I suggest ordering your parts from iFixit.com, there are a lot of websites/vendors out there selling what is misrepresented as "OEM" parts, but if you read their reviews you'll get some horror stories of parts not fitting, and not being recognized by the systems being repaired. Stick with iFixit.com!