Hello from my recent experience, I think manual installation gives you the possibility to choose the components you want to install for example you might not need Neutron at all, while DevStack installs OpenStack using sensible defaults. One thing I was explained here is that on the hypersivors point of view, DevStack installs a KVM as opposed to Xen that you might want to use of VMWare etc.

I hope this helps

I have an experience of 2 months on Openstack. This is what I have found.

If you are new to Openstack and you don't have the patience to read the documentation of Openstack installation then Devstack is a good way to get started. You'll gradually learn the dependencies among the different components of Openstack (keystone, glance, neutron, etc.) by playing around with them and also bit by bit by reading on docs.openstack.org . Its mainly intended for source development, that is, if you want to contribute to Openstack, this could be a framework to test your code.

1. You can easily use pdb to debug and trace the code respectively.
2. You don't need to have root access to modify the source.
3. It doesn't install the Openstack components as a service (service neutron-server start won't work).

Packstack is another installer for RedHat, fedora, etc. flavours. I don't know if it would work on Ubuntu. It is used for complete installation of Openstack, ie, installs all the components as a service in /etc/init.d.

Packstack can also be easily debugged but you need to access as a root to change the source.

ilearnstack is also a script to install openstack which I haven't tried but many of my friends suggest it to try.

You can use any of the above scripts to learn about Openstack.