We have used it in the past--some of the local grocery stores were offering free delivery and discounts off your purchases.
For the store we were shopping at, there was no markup in prices.
My thoughts:
-Useful if you don't have a car and want to stock up on heavy items (e.g. we used one of our free delivery/discount options to get a bunch of canned tomatoes)
-Useful if you are prone to impulse buying
-Minimum order amount--you HAVE to spend $x. If you aren't already consistently spending $x when you visit that store, maybe not so useful.
-Doesn't allow you to take advantage of random one-off specials. e.g. our grocery store often has meat marked down when it's getting close to its sell-by date. This is not something they advertise, you just have to be there in person at the right time. The instantcart shopper is not going to pick up the cheaper "manager's special" meat for you. Another grocery store we go to has a "bargains" produce section that operates on a similar principal--super cheap prices for stuff that is cosmetically imperfect or that needs to be used quickly. Again, a shopper isn't going to pick up on that for you. And on regular priced produce and meat, maybe you have particular things you like to look for, the shopper won't pick up on that either (say you are buying avocados for the week...you want one that is ripe now and one that will be ripe in a few days...but the shopper is just going to grab the first two avocados available).
We also occasionally had issues where we ordered something that was not actually available in store, and then had to have a text/call exchange with the shopper to let them know if there was an alternative item we wanted, which can be a hassle.