I do not have an SSD, but depending on how big it is, I'd recommend putting the OS on it and your most commonly used programs, i.e. the most common games you play if there is space to spare or perhaps a few office apps, i.e. outlooks, etc...
I simply feel the typical practise of putting the OS on the SSD is a waste of time.. After all, how much of a difference a few boot seconds make.! Using an SSD as a game drive makes more sense as games tend to have the highest amount of load times (much more compared to OS boot times).. For a non gamer, an SSD makes sense to use with applications such as used for DCC(s) when working with heavy file sizes for quick file access and swifter usage experience..
Heavy application like others mentioned, games, servers, IDE's and all those stuffs which have frequent storage interactions. But seriously as Emperus says, how much of a difference does a 2-3 seconds make in the boot time?
(1) Operating system + applications + games (Most commonly used). Set virtual memory to HDD, Disable Hibernation.
(2) On "speed" SSDs are in the range of 50 -> 80 Times faster in the important 4 K random read/write than a HDD and seek time is 100X better than the Fastes HDD (even in Raid0 conf). Boot time is much improved, not by just 2 -> 3 Sec. Post is not effected, but from Post to loading first program is about 20 Sec total on my I5 (intel 80 Gig SSD. How important is this, Depends on how often you boot and if you want to get that cup of java while booting. Programs load faster (very importan if working with large spread system files, editing photo. Games, not much in overall performance (Not a gamer), But any map loads will be faster. Internet, caching is faster, but more often overall speed is largly effected by ISP. Because of this I set my Internet temp files to a HDD (except on my Laptop - Only have connect for One SSD or HDD).
For laptops, they also increase battery time and lower temps. As long as you pick one of the lower power versions, some SSD's consume as much power as some HDD.
One it can be used like a normal drive ware most of the user content is stored on an external drive which is a common practice for those who have laptop and tablet pcs. Second it can serve as a normal boot drive ware internally there is one more more hard drive storing user content and programs. Third is a Raid 0 setup ware two more more SSD drives are combined for more storage and performance. Another option although rarely explored is to use a normal drive for boot and storage while the SSD is used for the pagefile alone which will still enable a noticeable performance gain in apps such as CS4 and photshop. Games will greatly improve especially when fraps is in use recording to the normal drive with out being bottlenecked by the bloated pagefile.