My DM and MIL looked after DS1 now 7, 1 day each a week, when I returned to work PT (2 days) when he was 11 months.
It worked really well for me, I had cover if one went away and my DS loved spending time with his GPs.
I don't think it worked so well for my Mum though and she withdrew her services whilst DS(2) was in the womb! She carried on looking after DS1 but said no more.
I felt really sad as I thought she enjoyed looking after him. Maybe she did but she now has to work.
I gave up work because I didn't want to put them in childcare, and although my IL's offered, they went on holiday for a month -the very month I was due to go back, leaving me without childcare.
I think if I did go back to working outside the home I would need to have a CM or after school club in place. As I would be left in the lurch by my in laws 8 + weeks a year on holiday! (which of course they are entitled to)
I think that grandparents sometimes offer without thinking if the commitment aspect of it. My MIL offered to have mine whilst I did my PGCE - then on further discussion she said well we'd work out between us - thinking my parents and sister would step in. And my parents won't, my DSis might but she is just about to have a baby so obviously won't be able to!
I am now a CM so I have no issue with Childcare!
It might be better to combine care with a CM or nursery - so that you have cover.
I am also of the same mind as a friend of mine who would rather have her parents occasionally babysit on a weekend, than have them all/ part of the week and begrudge it. That to me is more what I want a grandparents relationship to be like. And then I don't feel bad about asking for an occasional night off, drinking relaxing with the husband and not having to get up at 7am!