How does SQL Server Analysis Services compare to R?
In my opinion, it seems that SSAS makes more sense for someone who:


has significantly invested in Microsoft's technology stack and platform;
prefer point-and-click interface (GUI) to command line;
focus on data warehousing (OLAP cubes, etc.);
has limited needs in terms of statistical methods and algorithms variety;
has limited needs in cross-language integration;
doesn't care much about openness, cross-platform integration and vendor lock-in.


You can find useful this blog post by Sami Badawi. However, note that the post is not recent, so some information might be outdated. Plus, the post contains an initial review, which might be not very accurate or comprehensive. If you're thinking about data science, while considering staying within Microsoft ecosystem, I suggest you to take a look at Microsoft's own machine learning platform Azure ML. This blog post presents a brief comparison of (early) Azure ML and SSAS.