Software as a service (SaaS), also known as "On-Demand Software" is a service to customers via the internet. SaaS delivers over a standards-based network called the Internet. The pros are....
Drives innovations in security and reliability;
Softwares r always up to date;
If it is needed, it is possible to add functions & applications rapidly;
Even it supports the accessibility from remote locations;
If systems get spam targets, respond faster to threats and provide insight related to domain.
As far as my knowledge the most biggest problem is that once get all of the data in SaaS server, then it is not possible to get it back again. Although, some vendors are claiming that, they can do, but in real world and in practical sense NOT possible.
Though SaaS applications are a cheaper option over time, but most of them will charge per user, per month fee and even hike their rate time to time. It is known that, mostly of them provide a single user application, so it is difficult to customize in near future. If needs to pay recurring subscription fees then it is another issue. If possible, try bulk users utility to get maximum opportunity from this unique concept.
Before dealing, be care about the terms and conditions and if possible, read this thoroughly. And most essential part is that, be sure with 100% web based solution that does not require any installations onto the computer in future. SaaS is beneficial and reduces need for expert IT personnel to handle maintenance, monitoring, and software updates, if the vendor provides all.
Remember, SaaS is not the best choice for outsourcing.