I'm a few months into being a part-time working mummy now, and I have to say it is still working out pretty well for me.
The 2.5 days a week seems to perfectly suit our family. I get two full days a week at work, two days where I get to dress up in my nice clothes (those that fit me post-baby!), drink coffee uninterrupted, have adult conversations, and do my job, which I really enjoy.
I don't feel like I am totally back up to speed with work, due to a lack of sleep and getting used to managing my time when I'm only there 2.5 days a week, but I'm getting there and thankfully my manager has been fantastic about letting me ease back into work slowly. I envisage things would be different if I worked in a high-pressure corporate position, but I work as a librarian in a children's charity, so it's pretty nice.
I also get two full days just me and my pig, which is awesome. Unless it is a day like today when she is a grumpy little madam of course, in which case I'd rather be at work, but MOSTLY.
2.5 days to me seems like the best of both worlds, it's just fantastic, a great work-mummy balance.