sminlal is right, generally you should distuingish sequential I/O from non-sequential or random I/O.
For mass-storage, you only need sequential I/O performance. You won't be booting or launching applications; that's something for the system drive which focuses on random I/O instead - and would ideally be an SSD.
Let me explain the difference between random I/O and sequential I/O with some graphs. Compare the Velociraptor (generally the fastest consumer HDD) to the SSDs:
Hey the Velociraptor does pretty good here, and its even faster than the X25-M G2 Intel SSD. Sequential I/O is something HDDs do very well, so its ideal for mass-storage of huge data that is not 'executable'.
But, when we look at random I/O, we can clearly see the weakness of HDDs. This is most important for the system drive, things like launching applications and booting:
The Velociraptor looks like a floppy drive compared to the SSD's here. That's why HDDs can slow down an entire system if it has to seek alot. The throughput drops below 1MB/s. Nice you can do 100MB/s+ when copying files, but that doesn't work when launching applications, etc.
So SSD's are extremely good in random I/O making them suitable as a system drive for booting and launching application/games. HDDs are still useful for mass storage, and are pretty fast in that as well - competing with the SSDs.