Because we're damned if we do, and we're damned if we don't!
UA-Tester [1.06 Release]
code.google.com –> http://code.google.com/p/ua-tester/
Note: The 1.06 [purple pimp] release is now available in Backtrack 5 (http://blog.c22.cc/2011/05/16/uatester-now-in-backtrack-5/)
The results returned detail only the changes between the initial reference request and the new request (with altered user-agent string).
For full output us the verbose setting to return the complete response headers for each request.
Further example screenshot (wordpress.com):