Many people using ADFS, but a growing number are moving to Azure Active Directory. This is a little trickier since SharePoint only supports SAML 1.1 and Azure AD only supports SAML 2.0. However there are ways to work around that by using an intermediary IdP.

The answer to that is always going to depend upon the requirements of your organization. However in most cases I prefer going with provider hosted apps, unless there is a really good business justification not to do so. It requires significantly less customization of your infrastructure, and is also much easier to develop for, upgrade, etc.

I'm not going to lie to you, it's not a walk in the park. However I've built some additional extension classes for TokenHelper that comes with Visual Studio and abstracts out most of the difficulty of developing apps for SharePoint farms that are secured with SAML authentication.

It's easy to have your head spinning trying to stay on top of all of the different app models. For now at least, there are some fairly obvious uses cases for each with little overlap. The best way to figure this out is to look at your application requirements and then we can talk about which APIs are needed and appropriate for each piece you need to implement.

If you are using SharePoint on premises, and you are securing it with SAML authentication, then the answer is an absolute and resounding YES! It is critical for basic functionality and usage that case. I've written a number of these and have multiple samples available up on my blog that can be used as a starting point.

Absolutely! I've developed quite a bit of code and content around Yammer integration. We can use that to pull Yammer content into your applications, or vice versa - to push content from your LOB apps into Yammer. Of course we can also hook in other social appliation data as well, such as Twitter, Facebook, etc. The CloudTopia application on the Share-n-Dipity blog is a good example of having done that: http://blogs.technet.com/b/speschka/archive/2014/08/11/cloudtopia-connecting-o365-apps-azure-and-cortana-part-4.aspx.

The classic answer is "it depends" of course, but generally speaking getting search results from Office 365 in your SharePoint farm is generally not too difficult. There are several moving parts that are required, such as getting your Office 365 tenant set up, getting directory synchronization established, getting federation or whatever your authentication choice is going to be set up, and configuring the trust between your on premises SharePoint farm and Office 365. The bigger impact typically comes when you have a large organization and you may need to request the involvement of several teams to get all of these various pieces configured.

Sure thing! If you browse my Share-n-Dipity blog you'll find posts on a variety of topics. That includes Azure, Windows Phone, even Cortana and voice recognition!