Hello Phann,
Ok, I understand. To avoid unnecessary duplication, I will provide links to respective sections of our website:
Product benefitsFeaturesSecuritySelf-hosted serverAs for fastness and easiness to use - our users judge about that, not us. Unlike some of our competitors we do not shout from the rooftops about how great we are :) We just do our job.
Among the current disadvantages are the lack of Mac OSX Support and full-fledged address book in the mobile client. We hope this year we'll implement both of these.
Hope that helps.