Author Sayontan Sinha has committed many hundreds of hours to crafting a theme that really will meet the needs of novices and pros.
Out of the box, Suffusion is an attractive theme that can be quickly enhanced with included widgets and colour schemes without delving into the full array of customization options available.
This allows beginners the freedom to start with the simplest steps and progressing into more and more customization as their time and interest allows.
For pros, there is basically nothing this theme cannot do.
For fairly common customization wishes the wide range of "click-and-set" options makes the work easy. Still, if you want or need to do very specific customizations that are not included in the various "control panel" settings, the Back End panel allows you to add custom code to heart's content without having to FTP up/down or directly edit any files.
Using the Back End custom code feature makes testing out your css tweaks easy. Working with two browser tabs, one with the Suffusion custom code panel and the other your site, you simply save your tweaks, switch to the other browser tab, reload and see how they worked out.
If you are working at that level of customization I also recommend using Firebug to help you identify elements and test them out in Firebug before committing to Suffusion. But no matter really, because undoing your changes is as easy as removing them from the custom code box.
Long time WordPress admins know that there is one very critical consideration in adopting themes or plugins. Stability.
Stability does not just mean the code base is stable, but often more importantly, that the development base is stable. Sites get themselves into all kinds of trouble when they whole-heartedly adopt some theme or plugin and then a couple of years later, the author abandons the project. The code base becomes vulnerable over time or just non-functional with new WordPress releases and unpleasantness ensues.
While Sayontan is clearly a committed developer and shows no signs of walking away from his projects, he has done better by us than mere personal commitment. He has created a community of users that includes many who work with the code on a regular basis.
Face it, things happen. People's lives change or catastrophe strikes. So it is vital that mission-critical code has a succession plan.
With such a large community of users and Sayontan's willingness to include all willing hands in the development process, I am confident that should the author sign on for the first Moon Colony, Suffusion will survive and thrive.
Short version: Suffusion is the best theme available for all purposes and it is here to stay.