I recently bought a Scag V-Ride stand-on mower with a 61" Velocity+ deck and a Kohler 26hp EFI motor. It is a 2015 model with about 220 hours on it and was in excellent condition. It is a full blown commercial mower and I got it for the same price as a residential grade Scag, Exmark, Deere, Toro, etc. mower.
Here are the advantages:
1. They are shorter in overall length, so are more nimble and fit into tighter spaces than a ride-on zero turn or lawn tractor. So, there's less trimming to do with a string trimmer.
2. They are shorter, so take up much less room on a trailer or in the garage.
3. If you get into trouble, such as a loss of control on a steep hill, you can just step off of it rather than having to hang on and ride it out like you would have to do with a ride-on.
4. On some of them, such as the Scag, the standing platform flips up and latches to the unit, so you can instantly turn it into a walk-behind to get under low hanging tree branches or to tackle a steep hill.
5. Most importantly for some people, you stand up and use your knees for shock absorbers rather than sitting on a ride-on and forcing your back to absorb every jolt and bump.
6. It is MUCH easier and quicker to step off the mower and pick up sticks and trash, and then hop on and go, than it is with a ride-on mower.
7. The deck is sitting out in front, rather than under you, so it provides better visibility, and it's easier to see what your'e doing to trim next to, and maneuver around, obstacles such as fences and shrubs.
8. Stand-on mowers have a lower center of gravity.
9. When operating on a side-hill, the operator can lean to the uphill side to shift the center gravity to the uphill side, which you can't do on a ride-on.
10. Finally, standers are typically a little cheaper than their ride-on counterparts.
The negatives that I can think of (so far, based on my limited use of mine) are:
1. The controls (at least on mine) are more touchy than my Grasshopper sit-down ZTR, so mastering fine control and making straight lines takes a little longer to accomplish.
2. During the initial learning curve, you have to be careful when you back up. If you back up quickly and let go of the controls to stop quickly, you can throw yourself off backwards. But you learn quickly not to do that.
3. The deck lift is via a lever rather than a foot pedal, so it's harder to feather the deck over tree roots or other obstacles.