With out getting into how many centrifuges, what kind of inpections, years to breakout, etc, we need to recognized a few basic facts.
The Iranian want their own nukes. They want them so badly that they will do anything to get them. Flim flamming inspectors, lying, breaking their sworn treaty, is small change to them.
Their first and greatest desire for nukes is to deter the Americans from doing a regime change on them, just the way they did to Saddam Hussein. Even the hardest of hard core American hawks will be deterred by a threat to nuke Tel Aviv, or Riyadh or Cairo or New York. Without nukes, the Americans could do Iran down as easily as they did Saddam. One division, 3rd Infantry Division, (although it had enough tanks to be called an armored division) crushed the Iraqi army in a matter of weeks. The Ayatollahs know that right now, the Americans could do the same thing to them. Only Iranian nukes give them a chance of survival.
Secondary desires for nukes are the prestige they would bring Iran, very helpful in asserting control of the middle east. And the really hard core Ayatollahs dream of nuking Israel.
We don't want the Iranians to get nukes. Soon as Iran gets a nuke, everyone else in the middle east (Saudi, Iraq, Egypt, Turkey, even Jordan, will get them too. The Pakis have nukes now, and would probably sell bombs or technology to fellow Muslim powers. So would the Norks. Soon as everyone has nukes, someone will use them. Leadership in that part of the world is full of crazies.
So we ought to be telling the Iranians, you don't get nukes. If you try, we will bomb your nuclear sites. And tighten the economic sanctions even more. Cooperate and we will lift sanctions. Cooperation means turning over all enriched fissionables and destroying all the centrifuges, and no you don't get a nuclear reactor.
Obama doesn't understand this. But the voters do. Even the Congressman do.