Mine works but I only get like 1.5 hours out of it now. I might have to get a new one soon. Or upgrade to the new one.

nope, still holding.

Mine seems fine as well o:

I have my 3ds since September 2011 and I never replaced its battery.
It does seem to last a little less than it used to when I bought it, something like 20-25 minutes less, but I really don´t feel any serious problems with it.
I bought a 3ds USB cable and an external battery pack that I use when on long trips or 3ds fans meetings (where we do use a lot of local play )

I replaced the battery of my first 3DS 1,5 years ago, because the battery life had decreased to 1-2 hours.
I also had to replace my first charger, because it died.

I guess it's bad for the battery if the 3DS is constantly on the cradle in standby.