At worst, way more work than any one human being can handle, be ready for 14-16 hour days and perhaps having to come in on your days off if you're low on the seniority list.
The product line has become so vast that if you are a high quality, thorough merchandiser a heavy delivery day at a store can easily turn into 7-10 hours just in that one store (expect to do 2-4 stores a day, plus perhaps having to pick up someone else's store depending on how short we are on people).
Managers are for the most part okay (they need to get every store in the region covered so I'm not totally unsympathetic to their plight) but can get manipulative and try to coax (or flat out demand) you into doing extra stores on days when there's no way you could handle such requests.
You need to be selective in which areas you want to work as some sales reps are A LOT better than others. They need to make a living too/are being pressured to sell, sell, sell but if you get someone pushing a lot of excess product into stores that can't move it out you'll be maneuvering product around the back room all day and catching an earful from the grocery manager about it.
Some stores don't allow pallets on the floor after certain hours which makes it near-impossible to do the job efficiently when you have other stores to get to or if the no-pallets store is not the first stop.
Avoid merchandising Hy-Vee's at all costs. Cramped backrooms, 1-3 day sales where they get bombarded with product, no employees ever want to drop your pallets from the racking for you (or try to make you use the forklift to do it, which we aren't to do for insurance reasons, too many accidents).
Poor route designs where you either have too little to do in your own stores and end up driving all across the Twin Cities covering extra stores, or too heavy of a route with multiple deliveries on the same day trying to navigate cramped back rooms.
Once you get a few years seniority you can probably get a route with decent days (Sun-Wed or Wed-Sat, you usually need around 5 years in to get a Mon-Thurs or Tue-Fri start and if you get one earlier than 5 years get ready to be forced in), but that's almost worse than working opposite a seasoned vet as the merch on your other side will likely be completely terrible, unable to organize, product in the wrong location, displays built wrong or not at all. All new merchandisers get 3 weeks of training but if you're the merch who's training them it's often an excuse to throw extra stores at you (because you now have "help").