Customers should also obtain the rollout plan for the software in writing, says Mankowski. Find out what the vendor's rollout capacity is, he says. Will it add 100 of your users per week? Per month?

3. Missing SLAs. Service-level agreements, such as those guaranteeing vendor response time, are a critical component of SaaS contracts, says Mankowski. Some vendors provide SLAs with the contract, while others charge extra fees for SLAs or don't provide them at all, he says. "If it's a business-critical application and you need five 9s uptime, you need to make sure that's covered in the agreement with your SaaS provider," Mankowski says. Also, contracts should stipulate penalties such as credits or givebacks if service levels aren't met, says Jeff Kaplan, managing director at ThinkStrategies Inc., a consulting firm in Wellesley, Mass.

4. Performance levels. Customers should clearly define software uptime and availability levels with SaaS providers in writing. Before entering into an agreement with a SaaS provider, customers should ask the vendor for a record of past performance levels, says Kaplan. It's also wise to ask about business plans and investments that the provider is planning to make over the next three to 12 months, including enhancements to service-delivery capabilities, says Kaplan.

Customers should also ask how they will be contacted if there's a service disruption, and they should find out how much time the vendor has to fix the problem under the contract, says Mankowski.

5. Defining uptime. SaaS customers need to carefully define guarantees around system uptime, says DeSisto. Most contracts call for 99.5 percent uptime or part of your money back for a month. "But what does that mean?" asks DeSisto. "Is that 99.5 percent of planned uptime? Does the vendor plan to be down eight hours a month? If so, which hours?"