Personally, I'm thinking that we've totally squandered the largely-succeeding sanctions. From everything I've read, the sanctions were actually starting to threaten the long-term survival of the hardliners in Tehran. The mad mullahs were on the verge of being forced to make major concessions – one of which, certainly, would have been to stop supporting al Assad in Syria, the rebels in Libya, Hezbollah, Hamas, and the other external terrorist operations it's running. With this deal, or the parts of it that have been so far disclosed, we get an unverifiable (and therefore useless) commitment for a short-term hiatus in their nuclear development program. We don't get to inspect nuclear facilities on military bases at all – a hole in the fabric of the agreement that makes it obvious what Iran will do. In return, Iran gets ... essentially everything they wanted, including the end of all sanctions (including arms).
Why did we do this? The easy answer is “To burnish Obama’s presidential legacy.” The progressives are crowing, but their reasons all seem to be about appearances and a victory over conservatives. I have yet to see a straightforward presentation of the reasons for this deal, other than the Chamberlain-esque “peace in our time” platitudes, which anyone with even three or four functioning neurons ought to know by now is indefensible.
I think the chances are very small that Congress will be able to muster the two-thirds majority to override Obama's already-promised veto. Mad Mullahs: 1. Us: 0.