Reliability is often the most important performance criterion for a user: The application must be available when the user requires it. Note that this doesn't necessarily mean that it is always available, although there are some applications - such as the public Web presence of a global corporation - that have "24x7" availability requirements.
The impression of reliability will also be heavily influenced by what happens (or is expected to happen) in cases of unavailability: Does the user have a possibility to build a workaround? In case of provider problems: Does the user have someone competent to call - or can they even be sure that the provider will notice the problem themselves, and fix it in due time? How is the user informed during the outage, in particular concerning the estimated time to repair?
Another aspect of reliability is the predictability of performance. It can be profoundly disturbing to a user to see large performance variations over time, even if the varying performance is still within the required performance range - who can guarantee that variations won't increase beyond the tolerable during some other time when the application is needed? E.g., a 10 Mb/s throughput that remains rock-stable over time can feel more reliable than throughput figures that vary between 200 and 600 Mb/s.
– Main.SimonLeinen - 07 Apr 2006