Cannot say I've experienced any HDD related 'hitching' nor evidence that TW2 even streams much data from the disk. There are a few loading screens in Witcher 2 but they're 2-3 seconds long at best, an SSD might make them pretty trivial but that is a trivial 'gain' in the first place, saving 2 seconds?
There's an icon in the lower right hand screen that shows up when data is being loaded, and this is during gameplay. Depending on how fast your storage is, it could result in a slight 2-3 second pause or it could be like nothing happened if you're on an SSD. The first time I played the Witcher 2 back when it launched, I installed it on twin 600GB Raptors in RAID 0. To me, those pauses were annoying as they broke immersion.
Just a few days ago I installed the Witcher 2 on a Samsung 850 Pro 1TB SSD, and those pauses I experienced when I first played the game are completely gone, a long with the pauses from the finishers...
Maybe I'm looking at something else entirely but I just ran Witcher 2 for ~15-20 minutes running around dense areas and suddenly spinning the camera around on purpose, HDD usage never went over 10% during gameplay, some areas there's a little 'loading' without the screen, just the Ouroboros on the bottom left, only at one moment like that it went upto 30% but that's momentary just 30. So I personally haven't seen TW2 to be a case where the HDD is choking yet. This was(on purpose) done on SATA2 instead of 3 and that didn't make a difference in this case.
It's possible it may be settings dependent. I've always played the Witcher 2 on maxed settings (minus ubersampling), and so those settings could impact things like draw distance, texture size etcetera. Doubtless those two factors influenced how often the game accessed storage as well.. One area where I remember those pauses were though, was when you did a wide circuit run of the forest outside of Flotsam. Start at the southwest entrance to the Forest, go to where the Troll bridge is, and then make a beeline straight to the southeast entrance and that prompts that loading icon to pop up.
That said though, this could be pretty subjective. Everyone has different tolerance levels. Personally, I hate pauses and stutters in my games so it bothers me..
Maybe Watch_Dogs would show some difference since that's a game which constantly thrashes the drive for data but I don't have it.
I've never played Watch Dogs on an HDD, but I've heard that yeah, it's pretty bad. Another game that surprised me as well was Crysis 3. Crysis 3 apparently uses some kind of animation system for tree destruction, and the animations never load from memory. They always load from storage. So if you or an enemy shoots a destructible tree, it triggers that animation to load and since it's never in memory, it always loads from storage which can cause a short pause if you're using an HDD..
<~Oh, and this too!