i've just gone to work from 4 days a week, my mum looks after my DS on all 4 days from 7am until 6pm'ish..... i trust her implicitly to mirror my parenting.
It works really well for us, she is brilliant with him and is a great support for me and DH. Without her help i would not have been able to go back to work as childcare would have not made it worth our while.
I think the key is for both of you to be relaxed and flexible.
I found that i was very anxious for the first couple of weeks when i called everyday to find that he may have missed a nap or was being fussy with his food... which he NEVER was with me.
This was just a teething period where DS was learning to adjust, so if you do decide to let your mum look after your LO, give it time, give her a chance to find her feet/routine that can mirror yours as closely as possible.
For us the advantages far out weight the disadvantages.
I have to be mega organised because i work 4 days a week, sometimes 5, but i know he is well cared for, loved and if i'm a bit late from work i'm not worried about childminder kicking off!
Good luck with going back to work
HTH