Cinema 4D is a program I've always found intuitive and easy to learn and use. It's got a clear logical interface, a lot of useful and impressive built-in features, and whenever I've been stuck there have always been a number of extremely helpful online forums and communities to point me in the right direction. As I've tried more complex animations I've had to learn to use most of the features including the integral C-like programming language.
Blender is also well known for having a supportive online community, but its' interface is far from intuitive and it presumes a great deal of field knowledge - so why change?
Firstly, I've been using an outdated version of C4D but now want to try to produce some more advanced work, which means either upgrading or looking elsewhere.
Secondly, I have to acknowledge that cost is one issue. Most 3D packages are sold under a regular commercial software model, and even with the various upgrade offers, keeping up with the latest, most advanced versions and plug-ins isn't cheap for a non-professional user. Blender is different - it's an open source project on the Linux/GNU license model, so it's free to use. I should add that in my opinion C4D probably still represents good value to a commercial user.
Finally, I'm a little worried about becoming increasingly dependent on one software package over time - if I'm going to learn new systems then sooner is better than later.
Going through this change is making me think about the loyalty people have to computers or software - whether it's PCs and Macs, or more specific applications such as interactive fiction languages Inform and TADS, there are always users ready to defend their favourite package through reasoned debate (and the odd flamewar). I think this is only natural as, the more you use a particular program the more you are conditioned to its' idiosyncrasies and way of working, and switching to another program might actually be harder than if you were starting from scratch. With Blender it might be something as small as selecting with right-click instead of left, or larger jumps such as switching from COFFEE to Python. There's occasionally a near-religious feel to some of these debates though, and I wonder if, in future, mixed-OS couples will argue over whether to bring up their children in Windows or Linux.
Some first steps in Blender
I haven't quite abandoned C4D yet - I'm still working on two C4D-based projects.