I've briefly looked over what DNN has to offer (mainly through a spreadsheet I found comparing the various .net CMS systems).

I'd really appreciate any advice or information on these few questions:

1. How's the overall reliability of dnn, and is it relatively straightforward to put measures in place for backup/restore in the event of a failure?

2. How easy to use is it compared to other CMS' for non-technical (or limited technical) users? The main tasks I forsee my users needing to perform would be WYSIWIG text editor based changes (modifying text, images, links etc.), managing photo galleries, and updating blogs and latest news systems. I believe all these modules are available

3. Is it simple to move/clone a dnn site from a staging server to a development environment?

4. How does dnn compare to other .net CMS' in terms of website security - does it need anything out of the ordinary in terms of permissions etc?

5. How flexible is dnn in terms of templates/skins? The CMS I've currently been using is completely flexible - you can code your master page templates as you would a regular asp.net site. I'd trade some flexibility for reliability, but I'd appreciate any insight on how flexible dnn is. I've had limited experience with Joomla, and found the template/skin system there to be quite complicated and restrictive compared to what I'm using now.

A little background to explain my questions - I've built a few sites using the Umbraco CMS, and while I've been very impressed with it I've run into a few issues which have prompted a re-evaluation of whether it's the best tool for the job.

I'd really appreciate any advice or information on dnn - thanks.

1. I agree with Jeff's response.  If you know how to do a generic backup of files (ZIP and copy) and perform a backup/restore of a database then you're good.  Many hosting providers even provide automated tools for this.
 
2. WYSIWG is used in some modules.  Your ease of use will depend largely on the particular module and the knowledge of the user.  For instance, if you stick with the default DNN modules:  Basic text editing is very easy thanks to WSYWIG.  Creating a simple photo gallery (different module) is fairly easy.  Creating blogs with the BLOG module for multiple users is somewhat easy.  Creating a file repository with the REPOSITORY module is....er,...let's say, "not so obvious" the first time you try.
 
Any piece of DNN that you'd like improved probably already has a third-party module written to do just that -- well over 7000 modules and skins have been written for various versions of DNN.  "Ease of use" will vary greatly depending on what module choices you make for particular tasks.  View this as a blessing -- the alternative would be to have no choices at all.
 
3. I'll respectfully disagree with Jeff on this point.  If you've used simple "build-and-deploy" tools to create non-database-driven websites then you'll be dissapointed to learn that DNN is not so simple to move around.  Much of what you "build" is actually database data rather than file-based data.  You therefore don't simply copy files from your PC to a server when making updates.  You can move DNN from one box to another but you cannot easily "push" updated content from a local copy to an online copy.  Your users are contantly altering the database simply by using the site.  If you did a "push" from a local copy to an online copy you'd wipe out data (logs at the very least).  Typically content editing happens on the "live" copy or, alternately, on a local copy and then the content is copy-and-pasted module-by-module to a live copy.  There are some exceptions but genereally this is how it is done.
 
The "best practice" for managing content on DNN is to employ modules with "workflow".  Such modules allow users to create "draft copies" of content that can be reviewed, approved and "published" by a content admin.  Often such modules will include some sort of content revisioning so that you can "go back" to an older copy of content if needed.
 
4. As Jeff said -- it depends on the admin.  By default, DNN is fairly secure but you can easily make it less secure if you misuse / misconfigure it.  For better security than the default you'll want to get a security certificate (costs money) and apply it to your whole site to encrypt all pages/data passing between the client and your DNN installation (same as you'd do for any web platform).
 
5. Again as Jeff said -- it's pretty straightforward as far as skinning goes.  Skinning in DNN comes down to knowing the DNN skin objects, knowing standard (cross-browser) CSS and learning how to apply desired styling to whichever DNN menu system you pick for a skin.
 
 
Regarding the rest: 
DNN has limitations just like any other web platform.  When you chose to do something a certain way it naturally makes it more difficult to then do certain things in other ways.  Joomla is a perfectly adequate platform but the third-party module scene is not as mature as you'll find with DNN -- for instance, you can run into situations (much too often, IMO) where certain third-party modules interfere with the basic operation of other third-party modules.  Expect limitations from any platform.  I recommend that you go with whatever major platform most closely matches your knowledge and skillset to admin / develop for.
 
 
Good luck!
-mamlin