SSDs have no mechanical parts and therefore they are a lot less likely to "fail" then a mechanical hard drive, although the jury is still out on just how much more reliable they are.
But there are other factors involved for SSD reliability. Unlike magnetic hard drives, SSDs have a finite lifetime beyond which they won't operate. This is composed of two limitations:
- the number of writes done to the drive. Each write "wears out" the flash memory cells a little bit. SSD's do "wear leveling" to ensure that the wearout is evenly distributed. As an example of what this limitation means in practical terms, Intel claims that their drives will last "at least" 5 years if you write 5GBytes per day to them.
- the longevity of stored data. Bits are stored as static charges in the memory cells, and these charges dissipate over time. The information I've seen suggests that you should be able to rely on reliable retrieval of data for something like 10 years.