It is likely to be finalised/ signed into an agreement in June 2015.
The spirit of the preliminary understanding is “nothing is agreed till everything is agreed”. Though the probability of evaporating this understanding remains, the chances of conversion of this understanding into a binding agreement are bright. Give it rating of 60 for going through and 40 not going through. But the world is going to pay a very heavy price after conclusion of this pact as danger of wider conflict will amplify, though Obama will be gone next year.
The critical issue that US has achieved is this--Apparently Iran has agreed to demolish its nuclear capabilities of Plutonium/Uranium based enrichment of weapon grade fuel in three places Araak, Fordow and Nanatz. Iran has been in denial mode so far for existence of these facilities.
The fear is that Iran is in the habit of hiding more than revealing. If IAEA suspects that something fishing is going on, sanctions can “snap back”.
Saudis, Israel, Republicans and powerful Jewish community have been vocally against the agreement. Saudi/Yemen open war and proxy wars in Iraq, Syria, Lebanon with ISIS backing Sunni -Islam vs Shia-Islam can be very painful and can expand to other regions as well.
Saudis are openly talking about hiring Pakistan for nuclear and physical military support. Pakistan is in dire need of money. Overtly or covertly Pakistan may give in directly or through third countries. Iran will then have to prepare itself for nuclear deterrence. Unless the Middle East peace is restored, Iran-US agreement will remain brittle and edgy.
India has to side with Iran because it wants access to Afghanistan through Iran’s Chabahar port to assist Baluchistan to fulfil its agenda to keep pressure on Pakistan.India's NSA Mr. Doval has openly talked about assisting Baluchistan.
Fall in crude prices due to increased supplies from Iran is good for India but that will make Indian agro exporters more miserable.
On Trade and investment in Iran I will write separately