I went to upgrade to 10.9 and the system complained that the hard drive needed repairing. Using Disk Utility, I could not get the drive to repair or even format without complaint. Using fdisk allowed me to blow out the partitions and I was able to install but the OS was very sluggish. The drive was obviously in bad shape. The local certified Mac store wanted to charge $120 in labor. Doing it myself seemed the best option.
Swimmingly. The instructions were so easy to follow that I was able to do the hard drive swap while on a conference call for work.
Make sure your data is backed up. Go slow and take your time.