SSDs are better suited for either high bandwidth tasks or tasks that make a lot of small requests. HDDs at this point are better suited for lower bandwidth tasks or those that make fewer, larger requests.
OS and application programs often work with SSDs better because they make a lot of smaller requests (often under 100KB). The extremely small I/O overhead will do wonders here (0.01ms is a lot faster than 20ms in computer time). Pictures, documents, music and movies do not benefit from an SSD because they are rare, low bandwidth requests. Even a Blu-Ray quality movie only needs 4.5MB/s.
Games float somewhere in the middle. In my experience though, games tend to do bulk loading in rare instances. And while some games do benefit more from an SSD (I did a topic
on this), I usually don't care because my play time to load time is pretty darn large. But in the end, it depends on the game and what it has to load. For instance, I like to keep Skyrim on my SSD because of all the mods I have which eat into the I/O request time. But for something like Crysis, that can stay on the HDD.
Plus I don't know why everyone's so uptight about lowering load times. A minute between every hour or two of gaming is a nice breather.