An SSD uses less power and runs cooler.
However, I had a hard time justifing the cost and I am very happy using an > HGST Travelstar 2.5 Inch 500GB 7200 RPM SATA II 16 MB Cache Internal Hard Drive in my Mac Mini.
While a 7200rpm drive is not as fast as an SSD, it does boot faster than the OEM 5400rpm Hard Drive.
Plus I got way more GB for my money, which allows me to keep more media on my main drive.
On the hardware side, SSDs have no moving parts and
are therefore somewhat more reliable than HDDs.
As far as everyday use, the benefits of an SSD are dependent
on what apps you are running on a computer. Apps that are
very file intensive will see the most benefit as they can access
the data files much faster. In average use however, web browsing,
email, video streaming, there will be no noticable benefit in speed
other than the speed at which apps can open.
Depending on how much you currently have on your system, adding
RAM will likely give you a much greater performance boost for every
SSD, other than speed and boot time are NOT worth it
If you read boring tech specs on SSD, they are genuinely psychotic 'storage' (i use that loosely for SSD) devices.
SSD belongs on macbook Airs, and ultrabooks, everything else, forget about it.
SSD have no moving parts......which makes them more reliable, .....but theyre also like storage dynamite, and thefore ultimately MUCH LESS RELIABLE than standard HD.
The Fusion Drive that came in my Mac Mini from the factory is a combination of a 128GB SSD and 1TB HD, but OS X treats the two as a single device. OS X has long had a feature where the most active files are moved to that part of the drive that has the fastest access times. That offered some improvment in disk I/O. In the case of the Fusion drive, the fast part of the drive is de-facto the SSD portion of the combination. In theory the Fusion Drive provides the speed of an SSD with the relatively low cost capacity of the HD. In practice, and after some runtime experience where the OS measures file activity and moves the most active files to the SSD there is an obvious improvement in overall system performance and the cost is bearable.
Kevin Fromcamarillo mentioned the OWC upgrade and IMO it is a good one. There are articles around that will tell you how to format and configure an SSD and HD to be a Fusion drive. I have not tried that so I cannot say whether it is as good as the Apple factory Fusion drive or not. Personally I think the Fusion drive concept is like a hybrid automobile. An intermediate step until technology brings the cost down and capacity up enough to make them fully competitive. That time is coming sooner rather than later. Notice Appe is not even offering HDs on some models and instead going to all SSD. I don't think it will be too long before I am tryng to move my Fusion drive into an external enclosure and replacing it with a high capacity SSD.
A caveat with the Fusion drive. If you use third party volume repair and maintenance utilities, be absolutely sure the version you hvae is certivied compatible with a Fusion drive. Even when apps are certified compatible, that compatibility may not indlude all of their functions.