One way to look at the preliminary deal concluded with between the major powers and Iran is whether it is good for the participants, which in this case are effectively the United States and Iran. (The other major powers either have interests that are substantially similar to that of the US (Britain, France, Germany) or have interests that are a mix of US interests and a desire for a peaceful resolution which does not involve the US taking military action (Russia, China).)
"Good" in this case is an objective question the answer to which is determined by whether the agreement advances the relevant country's interest. It has nothing to do with fairness, law or the international community.
With that framework in place let's go below the fold and discuss American and Iranian interests.
The United States is interested in (i) preventing Iran from obtaining a nuclear weapon; (ii) preventing Iran from gaining significant influence over or destabilizing Persian Gulf countries; (iii) reducing the ability of Iran to project power and support its allies in Syria and Lebanon; (iv) a lower oil price; and (v) changing the Iranian regime to a more friendly one.
Iran is interested in (i) maintaining its current political system; (ii) a higher oil price; (iii) deterring military attacks on Iran; (iv) supporting Iranian allies in Syria and Lebanon; (v) supporting Shia communities elsewhere in the Middle East; (vi) becoming the dominant power in the Persian Gulf region and in particular being in a position to exert influence over Iraq and Saudi Arabia; (vii) being recognized as a significant regional player and (viii) obtaining nuclear weapons.
Although the addition of any country to the club of nuclear armed countries is a negative (as the simple increase in number increases the danger of a horrible result), Iran has a history of supporting terrorist organizations and actions and the former President of Iran was a dangerous nutcase who was intemperate and irresponsible in his speech, the odds that Iran would actually use a nuclear weapon on the United States, Israel or the Saudis is extremely low. From the American point of view the reasons an Iranian nuclear weapon is a negative is that it restricts American freedom of military action, may motivate other countries in the region, primarily Saudi Arabia, to seek nuclear weapons and may embolden the Iranians to more aggressively support Shia communities in Saudi Arabia and Bahrain.
America's interest, therefore, is not sufficient to motivate or justify extensive military action. And, it is important to note here that to prevent Iran from obtaining nuclear weapon by military means would require extensive military action, either a full scale occupation or a significant, sustained bombing campaign which would likely need to be repeated and would likely fracture the current consensus supporting sanctions against Iran. We could probably do this if we really had to but it would be expensive, messy, and dangerous. To put this a bit more succinctly: there really isn't a viable military solution.
Given the absence of a military solution, the analysis of whether the agreement is in America's interest is simpler. The agreement makes it more difficult for Iran to make progress toward a nuclear weapon and, in some respects, most notably by requiring Iran's most highly enriched uranium to be diluted or transformed, pushes Iran away from doing so. In exchange we ease some sanctions on Iran - a deal which is reported to be worth $7 billion to the Iranians. Most sanctions remain in place, but this deal improves the Iranian position in some ways.
The question then is whether, on balance, this is a good deal for the United States. One way of looking at this is to imagine what would happen if this deal were permanent. In that case, Iran would remain under crippling sanctions and never obtain nuclear weapons. A second is to note that it significantly advances one American interest (preventing Iran from obtaining nuclear weapons) and does not compromise any of the others.
The result in both cases is the same: this is a good deal for the United States.
It is also important to look at this from the point of view of Iran and consider whether it is a good deal from the Iranian point of view. This diary is fairly long however, so if there is interest, I'll post the Iranian point of view as a follow-up.