I am a college student and I use my Mid-2009 MacBook Pro on battery almost every day I have class. It usually lasts me all the whole school day but the charge capacity has gotten lower and lower in the past few months and after looking, I had gone through 600+ cycles. I also got a "replace now" warning on my battery icon in OS X. So, I figured it was about time to replace.
Easiest time I've every had replacing an internal component. Just take out the access panel screws. 3 screws to take out the battery, unplug it, plug in the new one, 3 screws back in, and back goes the access panel. Clean and easy!
Apple says the battery on my model of MacBook Pro it not user replaceable. So, if you replace the battery yourself while your MacBook Pro is in warranty you may void it.
But for me, my MacBook is years out of warranty and it was an absolute breeze to replace. I think just about anyone can do it. I also recommend you get the battery removal tool here on ifix. if you try to use something else you may strip it like I almost did.
Also, you can drop off your old battery at any Apple Store and they will recycle it for free!
Phillips #00 Screwdriver
P6 Pentalobe Screwdriver 2009 15" MacBook Pro Battery
MacBook Pro 15" Unibody (Mid 2009/Mid 2010) Replacement Battery