I bought the stump grinder when I first got my PT (shipping cost for implements alone is too expensive).
I haven't kept count but I'm sure I've removed at least a hundred stumps, some up to at least 30". Well, MAYBE only 50... kinda like "how long was the fish you caught?". But even 50 is a lot. It has performed flawlessly.
The PT grinder will take longer than a commercial one, but it's not that big of a deal. If I had a 30" to remove I'd "budget" an hour. Maybe with a refreshment break, it could take a little longer.
If you do get the grinder, many of us have found it works WAY better when the cutter is reversed. That is, as it comes from PT, all the chips are thrown back at the operator.
Reversing the cutter causes the chips to be thrown away from the PT. I've used mine both ways and there is NO disadvantage to reversing it, unless you don't pay attention and toss chips and maybe a few stones onto your neighbors Mercedes.
I have only used mine for trenching in dirt. I have a trench I need to make across a paved driveway, but that's something I've been putting off for a couple of years now. Supposedly, that job won't damage the cutters but even if it does, they are individually replaceable. But the job isn't THAT important and is easy to postpone.
Phil