How similar are Hakka and Cantonese?
If we take Guangzhou to represent Cantonese and Meixian to represent Hakka we can get an idea.

Part one:

Isolated-word-intelligibility test




With Hakka as the speaker people from Guangzhou only understood 35% of isolated words.
With Guangzhou speakers Hakka listeners only understood 40% of isolated words.


Part two:

Sentence intelligibility test




Guangzhou speaker were only able to get Meixian listeners to understand 55% of spoken sentences.
Meixian speakers were only understood 12% of the time by Guangzhou listeners in spoken sentences.




The standard for mutual intelligibility works like this:


  If two languages are 80% intelligible (or more) with each other, then they are dialects of one language rather than two separate languages.


These two certainly don't meet the requirements.

It seems that Hakka speakers can understand more Cantonese than vice versa.



If you'd like to know more about the intelligibility tests you can check out this PDF here. Where section 3.1. Isolated-word-intelligibility test goes over table 2 and section 3.2. Sentence intelligibility test goes over table 3.