Most are using a SSD as the os drive. Large capacity SSD's are too expensive to buy for most. I have a 64gig C300 SSD as my OS drive. I only have about 1/2 filled with all the progams I use. My other two HDD are used for storing media such as games, mp3, videos etc.
It's also a very good idea to size the system partition on HDDs
to something like 30 to 50 GB, because this results in
"short-stroking" the READ/WRITE armature in HDDs
with rotating platters.
By segregating the system partition in this fashion,
the remainder of the primary HDD can be dedicated
to data files, and drive images of the system partition
can be archived on that dedicated data partition.
The next graph shows how data rate decreases
from outermost to innermost tracks, because
recording density is more or less constant
on all tracks: