There are so many variables when it comes to shipped chicks... as someone else listed above...
1. Your distace to the hatchery
2. Weather conditions when shipped (if it is January and 20 below than you may have a problem)
3. Hardiness of the breed ordered
4. Quantity ordered
Depending on the trip, they can be very tired/traumatized when they arrive and there's not much you can do. But if you add a little sugar or electrolytes to their water. make sure to dip their beaks so they know where it is, and give them the heat they need (as well as peace and quiet) if they haven't been taxed to the point of no return then they should perk right up. I ordered from MMH a couple of years ago. It was stupid on my part, because I didn't think about the weather... when I picked up my chicks at the end of February, it was only 7°F outside. I lost 4 within the first 12 hours, but I was new at brooding chicks and didn't know all the tricks. Now, I wait until at least March to ship them in, make sure I don't order them to be delivered around a holiday, let the PO know they're coming, and have the brooder/water all ready to go when they arrive. I haven't lost any chicks within the first 48 hours in the last two years, including 2 more orders from MMH. When I called them to let them know, they even gave me a credit for the two extra "peanuts", even though they shouldn't have been counted.
Ask enough people, and you will get reviews both ways. They wouldn't still be in business if they were in the habit of selling weak or sick chicks. Everyone's going to have problems from time to time, but overall they're just as good as any of the other large hatcheries.