It's been a good investment for many people for obvious macro reasons but also because:
1. You can live in it (thus realizing value directly).
2. Since you live there you can't easily panic and sell in a downturn.
3. Leverage (U.S. gov't FHA provides almost 30:1 leverage)
4. Lenders can't liquidate you simply on account of decline in value (no margin calls)
Even with all those points, you would in most cases end up with more assets renting something equivalent and dumping your down payment and mortgage in the SP500.
Again, the main advantage of a mortgage is that it forces you to save. If you are disciplined enough to save and invest in the SP500, you are way better off doing that.
