I interviewed at Persado in September. There were three stages, each conducted by a different engineer. It was relatively light on algorithms and data structures and focused more on short questions to assess your familiarity with the particular language (opposed to testing how you think as a programmer). I would consider the difficulty of the questions to be on the easy-side overall, although you are of course being compared against other applicants so that does not necessarily mean that it reflects on the difficulty of getting an offer. Each interviewer was very approachable and friendly, in their own way, and I would consider this one of the much more enjoyable interviews I've had.