US combat troops have left Iraq. Iran has fired up a nuclear reactor that is almost certainly the beginning of a nuclear weapons program.
What would the world be like if we hadn't invaded Iraq?
If we hadn't invaded Iraq, Saddam Hussein would still be in power. It's very likely that the UN would have let the post-Kuwait sanctions lapse. Whether he had chemical, biological, or nuclear weapons before we invaded or not, evidence suggests that he was trying maintain the ability to resume such work as soon as the sanctions and inspections ceased.
Today Iraq and Iran would be locked in an arms race. The most oil-rich and politically explosive piece of real estate in the world would be an even bigger basket case than usual. $4/gallon gas would be nothing if they started tearing up the Middle East.
Oil isn't the only reason we went into Iraq, and by itself it's not a sufficient one, but given than it is the fuel for every industrialized economy in the world, we should be glad that Saddam isn't in power today.