That would scare me, because subconsciously there would be be an expectation that I should be delivering the same as people who are working 5 days a week (and getting paid more than me). In some jobs, you'd end up doing nearly as much work for less money, although I appreciate it might be different in construction. The analogy I always fall back on is the coffee shop barista; if they have a day off work, they don't come back to a queue of people out the door and a backlog of drinks to make. But in IT, it sometimes feels like that.
Where I used to work, there was a lady who worked 3.5 days a week (and was paid as such). I was part of the leadership team that met to calibrate performance reviews and it was concerning that some felt she should not be considered "successful" in terms of performance rating. In my mind, if she delivered 70% or more of what a successful person working 5 days a week did, then she should also be considered successful or higher (that's not to say if she was rubbish she should get a free pass of course).
To be honest, in the past I've actually wanted to work more hours for more pay, e.g. go from 35hrs to 40hrs because I had more than enough work to fill that time. Essentially just get paid for some of the extra hours I was doing anyway.
