Is aegir a good choice for me? 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.