My DM has my children two days a week.
You asked whether it is hard to ask them to look after the DC in the way you would like - the answer is yes! You have no real control over how they are cared for whoever they're with, but at least at nursery there are procedures and standards that you can ensure you're happy with. I am lucky in that my DM raises the children in a very similar way to me, and we spent a lot of time together during my maternity leave, so the kids have always known DM and I work as a team. I still don't know precisely what she does/says to them all day though!
Does it put a strain on your marriage? Absolutely. My DM normally stays over the night before she has the children, but she arrives late at night, often when we've gone to bed. So we have an 'evening in' with DM maybe once a week. I think it's fine, but for my DH it is his MIL not his DM, which I know is more difficult. I hope our marriage will survive it, because the DCs really benefit from the days with their GM.
Does it work long term without causing resentment? Sort of! We've been doing this for about 18 months, and my DM says she's happy to continue. But we use a very flexible nursery for the other days of the week, so my DM is able to pick the days she comes with a few weeks notice. That means she rarely has to miss a social event, can do two spaces days or two days together depending on what she'd prefer, and can normally change days at fairly short notice if she wants to.
Does it work financially? It does for us as my DM doesn't want any money. I don't know how it would work if you paid your DM - as someone else said, you'd probably have to pay her as if she were a nanny, which would cost you and her NI, tax etc. it would also mean she is employed - would that affect pensions?
In conclusion, it works for us :) But we have a very flexible system, my DM is less than an hour away not 2.5 hours, my DM can pick her days, we only spend one evening a week with DM, DH and I together (not half a week) and we're all willing to put the children first when we get shirty with each other (which does happen). In your case, it sound like each of the things that I think make my system work will be a little bit tougher (less flexible, more travel time, etc).
For what it's worth, my DM was hoping to do a similar thing for my DSis. I don't think it is going to happen now, because DSis is 2.5 hours away an will have fixed days. My DM doesn't think she'd cope with the extra travel and organisation in her life (e.g. Leaving my DF a dinner on the night she travels, because whereas she can eat with him then drive 50 mins to us, she'd have to travel earlier to go 2.5 hours). I think it is a pressured system, and the extra travelling time and rigidity of days would make it too hard.
Good luck, whatever you decide :)