Definitely has a lot in common with Polanski's pre-America work, probably even more with his Polish debut Knife in the Water than with his previous British film Repulsion. That film also has an interloper of potentially criminal intent joining a young married couple in an isolated locale.
In this case, of course, we know that Dickie is a criminal, and it's actually the couple on the island who are the real wild cards. I thought that was a pretty interesting twist on film noir tropes, which this film uses a lot of - it's basically an even darker, even more unhinged noir than classic noir.
I liked the way that even though Dickie interrupted life on the island, it was already pretty weird there anyway, and a lot of the frustrations and outbursts that come up later on in the film are not due to Dickie at all (except perhaps as catalyst), but to Theresa's infidelities and contempt for George, and George's nervous insecurities. I also liked how Dickie underestimated Theresa basically all the time, even though he knew one of her secrets from the beginning. He thought he was safe because George was a wuss, but Theresa caught him by surprise several times. Her treating him like the butler (partially to explain his presence, but let's face it, mostly to humiliate him and turn the tables a little bit) was pretty dryly hilarious, and thanks to Lionel Stander's vocal range, gave me a bit of a William Demarest in The Lady Eve vibe.
But by and large, the film is pretty dark, and I liked that so much of it took place in the dark and Polanski and cinematographer Gilbert Taylor didn't try to make it look visible, but just let everything play out in shadows and half-light. It made everything that much more ominous, and gave the castle a physical and psychological presence that affected everyone in it. The music is also an interesting choice, as a lot of it is straight up '60s pop jazz, which isn't ominous at all, but provides an intriguing counterpoint to what's going on visually.