Salesforce is always available securely from any internet-capable device anywhere in the world, UNLESS you choose to set security measures so that ONLY trusted IP ranges may access the system at certain times of the day. It's all about choice and flexibility with Salesforce products.
They always advise of maintenance windows which never interrupt daily processes and I have only experienced 1 outage in my 5 years of working with SF which was resolved in less than 10 minutes.
All Force.com apps run on world-class data centers with backup, failover, and disaster-recovery facilities. Force.com has had a proven 99.9 percent uptime record for years. Accordingly, I only recall our instance of Salesforce having one unscheduled, brief down time over 6 years. I can't remember for sure, but it may have been due to our Internet Service Provider (ISP) versus Salesforce itself. Also, Salesforce does it's best to keep customers in the loop:
Trust.salesforce.com is the salesforce.com community's home for real-time information on system performance and security. On this site you'll find:
Live and historical data on system performance Up-to-the minute information on planned maintenance Phishing, malicious software and social engineering threats Best security practices for your organization Information on how we safeguard your data
On the last 12 months scale, I didn't encounter any unplanned outage or application errors. Sometimes the SFMC (Exact Target) is a little slow during noons, but it is understandable. Planned outages are adequately pre-notified on the login and via separate email alerts.
For an application this large and complex it’s very good. There have been a couple of down times, but it has never disrupted our business to a major degree.
There are only a couple of configuration changes that require them to take your instance offline and that is known downtime. An example, is when we turned on multicurrency.
Salesforce is a web-based CRM application that enables users to forecast revenues and track leads. It is a leading sales, service, and marketing app. Salesforce.com's Sales (CRM), Service, Platform and Marketing applications are designed to help companies connect with customers, partners, and employees in entirely new ways. According to the vendor, on average, reps using Salesforce see: a +29% increase in sales from greater visibility, +34% increase in sales productivity, and +42% increase in forecast accuracy.