NO, you should always layer your security. The developers of these products urge against running more than one anti-virus on a system due to the risk of conflict. Some folks have been working to configure these applications to coexist. From a security standpoint, in my honest opinion, this is a risky practice when you consider that application developers can make any number of changes when they release an application update or patch. Be prepared to backup and restore your system should something happen. I'd like to stress that an inability to connect the internet is a problem that plagues traditional anti-virus as well. At the very least, a cloud antivirus should offer local signatures and resident engine for scanning and removing infections. So unless a cloud anti-virus is strictly internet dependent the solution seem to be the same for both. Now let's say you do lose internet. At this point, your going to lose the "theoretical" benefits of cloud security, which is community support (i.e. sample submission/analysis, quick release of "more current" definitions, and access to any other services/utility server side). It's debatable whether these benefits are of substantial gain even when connected to the internet. Not to mention that traditional antivirus offer a lot of the same functionality anyways. These sales pitch is that cloud-solutions should offer these services at a quicker rate (i.e. your definitions should be more current against new threats, etc.). Sales pitch aside, its actually comes down to the methodology. What samples are these companies using? What samples are they not testing? etc. For example, faster definitions releases do not translate to better detection of high-risk threats. Not to mention, what do you and I define as high-risk? It's difficult to make a subjective analysis of these products without asking and answering these questions. To prevent a endless debate from ensuing, I will answer your topic questions: "How reliable are these cloud antiviruses?" Only as reliable as the definitions and engines they use to scan your system when connected and disconnected to the internet. This may seem like a cryptic answer, but what your asking really isn't a black and white, yes or no response. Do your research, layer your security, and accept the fact that there is no such thing as 100% security.