Why should I pay for an obfuscator if Visual Studio ships with a free version (Dotfuscator Community Edition)?
When I tried Dotfuscator Community Edition it worked fine except it did not include resource files.
Buying an obfuscator will give you access to support if you have any trouble with the obfuscating process.
As I wanted so save the approx. 1000 Euro too, I'm writing my own obfuscator that is in fact a c#-c# compiler.
Because Dotfuscator Community Edition uses only the most basic Obfuscation techniques.
Obfuscation is not a be-all end-all solution. If people want your code, they'll get it one way or another. Obfuscation just makes it more difficult.
Just check out the Dotfuscator editions page to see which techniques get used by which versions (and believe me, the PRO and Suite make your code MUCH more difficult to reverse engineer than the Community Edition):