Just replaced an Iphone 3gs Battery.

Disassembling is easy, the problem is reassembling.

Some tips:

Take your time on connecting the cables. When they connect, they connect. It's hard to connect them but once you do it right you'll feel it. I did very light tappings around the connector to see if any part was loose to check if it was all correct.

On the 3 cables from the logic board, connect number 3 first, it doesn't go all the way like it was before, that's ok AFAIK. After number 3, connect number 1 and then number 2. It worked better for me that way.

The battery even comes with a 20% charge, hehe.

Good luck to you all and have a lot of patience. I did it in one hour aproximately.