Here is a good place to start: http://www.dotnetnuke.com/Support/Get...

Here is a good matrix on comparing various CMS http://www.r2integrated.com/CMS-Compa...

Here is a good blog with some DNN tips and tricks http://www.r2idnn.com/Blog.aspx

Let me know if you have any specific questions, would be more then happy to help out.

 It all depends on what your requirements are and how you plan on using the CMS.   I have had sites were we can port an existing asp.net site into DNN without any issue.  If your only using the CMS to import data (admin) then your existing asp.net site can be altered to read from the DNN DB.  If your looking for full functionality, then you will probably need to port over your existing user controls into modules.  Before making a decision, I would take the time to fully understand DNN, what it can provide and also what your true goals are for the project.