Low costs and quick deployments seem to be making SaaS applications relevant. It does seem to be an inevitability that the "cloudification" of software applications will take over. However, for now one issue to be wary of is the level of service you will receive once you start outsourcing IT responsibilities.
This issue of SaaS vendors customer service is explore in this article about the SaaS dilemma between being a software or service provider:
Many consider this to be the 'next big thing'. It's been through a few trial rounds to get its feet, and is now being adopted by the big players. Worth considering as a short term investment, but beware of longer-term issues moving against the industry.