From my own experience, Cantonese can be mistaken for a lot of things. Some say it sounds like Thai or Vietnamese, while other times people mistake it to be Korean or Japanese (mostly Korean between the two).
1. I remember watching a Cantonese dubbed Disney clip on youtube, and someone had asked, and I quote, "What language is this? Sounds kind of Japanese-like"
2. I have also seen some non-Cantonese Chinese expressed disgust at the language and say it sounds like Vietnamese.
3. A popular shampoo commercial on youtube has been mistaken for Korean countless times (you may search "funny Chinese commercial" if you're curious).
4. There was also a Cantonese interview of Tony Jaa and the interviewer did mention that Thai and Cantonese have some similarities.
5. Lastly, an African American polyglot had been learning Cantonese and decided to make a video of himself speaking conversational Cantonese. A comment has been made that said it "sounds like Mandarin, Vietnamese and Japanese all in one".
As for me personally, I can't really tell what it sounds like. I've been speaking it since childhood, it's just too distinct to my ears to make a comparison with any foreign language. The opinions of people from various communities should be a good source of indication though.