I set my defrag to once a month and can't see any gain or loss in speed. There are those that think it matters and we have to let them keep thinking that as they tend to not want to use the machine but tinker with it all the time.
For example my neighbor's screen seemed to always be some defragmenter software for 5 years. I always wondered if they used the machine.
I bought a new HDD (Segate 300 GB SATA) last year (currently 90GB used). I use it to store all my files (have the my documents folder pointing to it). It is always fragmented! I defrag it and the next week its back to being very fragmented. Any thoughts ?