If you have a lot of JPEG, MPEG or MP3 Files, They are already compressed, so you won't see a great deal of change there.
If you have a lot of text files, documents, or programes, then it could make a substancial difference. Just remember, when compressing/decompressing it takes up a lot of your CPU, so files that you use often will be slower to open due to decompression.
Also to compress, the HDD needs to formatted in NTFS.
You can always try the compression, and if it's not working out for you then change it back. Really simple to do.
Click Start > My Computer > Right Click the HDD > Properties > Check compress.... OK
If not what you expected, Go back and uncheck compress...
Hope this Helps.
