SSDs have been generally available for a few years. As with most new technology they were relatively expensive when they were introduced. They are still relatively expensive if you look at the cost per gigabyte. Conventional hard drive technology has continued to advance, perhaps at an accelerated pace due to SSD competition, and for large volume storage they can not be beat.
Advantages of the SSD
There are several advantages that favor the SSD. Because there is no motor needed to spin up the platters the power consumption is less. Having no moving parts also means that they are more shock resistant. The major advantage, though, is the advantage of speed.
The primary speed advantage comes from the solid state design. With a conventional hard drive the major limiting factor has been the seek time to find the needed data. The read/write head is mounted on an arm similar to an old analog turntable (record player). The arm must swing to precisely the right position and the data must pass under the arm in order to be read. This takes a finite amount of time. The SSD simply calls the data from the virtual address where it is located.
With a conventional hard drive seek time can deteriorate over time due to data fragmentation. When data is written to the hard drive it must not overwrite data already on the disk. If there is not physical space available for the amount of data in a file the drive will write part of the file in one section of the drive and then find more space in another part of the drive. When data is called from the drive the head must move and the platter must spin so that all the parts of the file can be located. As more data is added to the drive more fragmentation will occur.
Disadvantages of an SSD
The primary disadvantage of the SSD is still the cost factor. The cost per unit of storage is still on the order of ten times that of the conventional hard drive. The costs have been going down for SSDs but they have also been declining for conventional drives.
There was quite an uproar when solid state drives were first coming on the market about the fact that the memory cells break down over time. Studies have shown that this is more of a theoretical problem than a real world problem for most users. Some applications are much more read/write intensive and can best be handled by a conventional drive. SSDs do have load leveling technology that insures even usage across the available memory cells to mitigate this problem.
When and Where to Use an SSD
Since one of the primary advantages of the SSD is speed the boot/OS drive will be a major source of advantage. There is more advantage with newer hardware, but even older hardware will feel more responsive if the OS and programs are on an SSD. The data rate for the interconnect to the hard drive can be a significant limiting factor with older computers.
A desktop computer can easily be set up with an SSD for the OS and programs and a conventional drive for storage. A laptop that has internet access most of the time can be set up with cloud storage these days so that a huge SSD is not necessary.
I recently built a new computer for my audio work. I put an SSD in that computer. The performance is so impressive that I have upgraded my personal computer and a netbook with an SSD. I will always use an SSD for the operating system and programs in the future.
Several years ago many computers were sold with just enough memory to get the job done. One of the best and easiest upgrades at the time was to add more memory. The prices of memory dropped and more computers were sold with, at least, a reasonable amount of memory. In older hardware today, if there is a good amount of system memory, the same kind of gains can be attained by migrating the OS and programs to a SSD. With older hardware the gains will not be as impressive but the computer will feel and respond much more rapidly!