Whether a company promises not to keep logs or not they can and always will comply with legal requests (court order, warrant, national security letter, etc, etc) to monitor specific targets. Otherwise they will be out of business.
Bottom line: don't do illegal stuff. These services (if they are actually telling the truth about not logging) are good for protecting your privacy as a law abiding citizen but they don't really provide any shield for criminal activity.
Bottom line: don't do illegal stuff. These services (if they are actually telling the truth about not logging) are good for protecting your privacy as a law abiding citizen but they don't really provide any shield for criminal activity.