I have worked pt for the last 4 years, starting at 20 hours per week.
I am in a managerial role, with a challenging and varied workload - policy writing, engaging with strategic partners which involves a fair bit of analysis, managing various projects, along with lots of queries which need an immediate turnaround. It is really interesting, but very demanding.
When I started pt hours, I had a really demanding boss, who disapproved of pt workers (he actually said this to me 
). He cut my staff without any consultation etc - I posted menny, menny threads on here. 
To summarise, it was crap.
I always ended up coming in on days off, working longer, bringing work home. I worked out I was working at least an extra 5 hours every week, unpaid, so after a year and a half, I asked to increase my hours by an extra 5 hours per week. I proposed that I worked 20 core hours per week, and the additional 5 hours were put into an annualised 'bank' of hours, which I could draw down from, and if I recall, I undertook that I would never be more than 25 hours in the red. This worked really well for me, and I would recommend suggesting this to your boss as a proposal. It means that you can be really flexible in relation to the extra 25 hours, which works well for both parties.
We then had a restructuring in work, and I got a new boss, who was, surprise surprise, also opposed in principle to pt workers. (Very male dominated sector). He gave me a hard time, and really bullied me with the intention of making me increase my hours. His argument was that someone of my grade needed to be there all the time. Absolute rubbish, and his argument was built on prejudice, not any hard facts about performance.
Anyhow, I resisted the increase in hours (also subject of menny, menny threads), and because of the bullying, a lot of the good-will went, and I made damned sure that I didn't work a minute more than I was being paid for.
Do you know what? I still managed to perform way above my targets, and deliver the work. I think you end up being able to prioritise better, delegate, and speed read. I find having a constant, never ending to do list really helps.
We had yet another restructuring earlier this year (we really like restructuring in our place) and as part of this, I had to prepare a report detailing the span of my role, and that of my team. It was completely evident that me & my team are completely out-performing the other units within our Department, despite me and some of my team being PT. The good news was that part of that restructuring meant that I got the most wonderful manager I could ever hope for.
I suppose the point I am trying to make is that necessity is the mother of all invention. When I started being really strict about sticking to my hours, I was still able to deliver on the work - I just worked smarter. And trust me, my job is anything but routine or uninteresting.
Hope this helps.