MITM attack is a serious security problem for public-key encryption systems. A mechanism widely used to avoid such attacks is the use of signed keys: if the key of User1 is signed by a third trusted part, which ensures the authenticity, User2 can be sure with a certain reliability, that the signed key is not an attempt of interception by a criminal. The use of signed keys, sometimes signed by a Certifying Authority (CA), is one of the main mechanisms used to secure web traffic (including HTTPS, SSL, or Transport Layer Security protocols).
Comodo will verify that the sites you're connected to, are using a trusted SSL certificate, however, the neglect of the certification authorities giving their approval to the correspondence between the identity information and the corresponding public keys, are an objective problem of these systems.
Not saying Comodo is not effective, but I think it is necessary to give some additional info.
