Based on the limited information we all have, myself included, I feel they have a 1 in 2 chance of pulling this off. The robotic subs are amazing in their capabilities. It's like watching a kid play XBox 360 - a kid that knows what he's doing. I think they'll be able to position it - it's just the high pressure will be an issue. Once the box is in place and they get the relief pipe operating, then the oil loss into the Gulf will go down dramatically.
Someone wrote they felt 15-20% of the oil would still leak out. If they can get good negative pressure on the pipe, it should be much less than that.