I want to use aegir for a website that users can create their own sub-sites.
From aegir installation guide:
when a new site is installed, the web server will be automatically configured (vhost) and restarted
Does this mean that every time a new subsite will be created, server restarts and will be out of access for a few seconds?
Aegir is a powerful tool to manage hosting environments, deploy new versions of your site, clone a live site to a testing environment... Stuff like that. I don't know which kind of users you're talking about, but I would only give access to experienced Drupal developers. If you want 'normal' end users of a site to be able to setup their own subsite, maybe http://drupal.org/project/subdomain is right for you.
The whole server doesn't restart when a site is added, only Apache, and it's just a graceful restart, so no real downtime..
Also, if what you mean by 'subsite' is http://example.com/subsite1, /subsite2, /subsite3, Aegir doesn't support that at this stage. Subdomains will work fine, but not 'subfolder' sites. See http://community.aegirproject.org/faq#Can_I_use_the_subdirectory_multisite_feature_of_Drupal_in_Aegir
