November 2012 I made the switch to Windows 8 Professional as my OS of choice on my HP XW6600 Workstation. This workstation has two Intel E5450 Xeon QC's inside and currently 8 gig of ddr2 ecc memory onboard.
For the installation I opted for the Intel SSD 330 at hand with the 6 Gbps SATA port and 120 Gb of room. That is for the OS, the desktop applications and nothing else. Data like documents, webapplications that I develop locally are on a WD Black 2.5 inch harddrive and a second Intel SSD 520 with 120 Gb is there for photo, video and audio editing.
I have read that you at least need to keep 20% free space at hand on a SSD to avoid loss of performance. How much is true in that case and how do I make the best use of my SSD's. I currently have 20.3 Gig of memory free on my C:\ drive. The page file is on this drive as well.
Free space is important on any SSD in order to maintain performance. Write performance is where performance is most impacted by a lack of free space, which can affect read performance as a side effect of write latency.
All SSDs have space set aside that is not accessible by the user, which is called Over Provisioned (OP) space. Intel SSDs, have more OP space then some other SSDs do, but it is still good practice to set aside 20 - 25% of a SSD's capacity as (user) OP space.
I'm surprised you have used about 100GB of the 330's capacity with just Windows 8 and your applications. You must use quite a few applications. A few suggestions to free up some space:
If you do not use Windows Hibernate, you can get back ~8GB of space (or whatever your DRAM memory capacity is) set aside for the hibernate file, by disabling Hibernation.
Reduce the size of the page file, or move it to another drive. Personally, I do not use a page file, I have it disabled. The page file is a work around for a shortage of DRAM memory, when PCs had 2GB or less memory. Unless you know you will be using more than your 8GB of memory, having a page file on your SSD is a waste of space IMO.
Do you run Windows Disk Cleanup to free up space? Are you saving old system Restore points that are likely useless?
OTOH, if you are not experiencing a loss of I/O performance, you don't need to worry about this very much, but I would not fill any SSD to capacity and expect it to perform as if it was empty.