To be fair, the parts I've had to purchase are consumables. Gage wheels/spindles, blades, deck belt, fluids, and the annual maintenance kits, etc. No breakdowns thus far. JD fluids are not any higher priced than aftermarket and there is guaranteed compatibility.
What aren't cheap are the accessories/attachments, but they have just about anything you could ever want to bolt on to your lawn mower - plows, snowblowers, 3 pt. hitches, rear PTO kits, cabs, sprayers, tillers, the list goes on. Makes for a very versatile machine if you only want to maintain one engine.