It is very fiendish. Apart from the first time I did it, "just one little line" does seem to end up being half the bag and staying up until 3 in the morning practically pissing myself with euphoria. This is quite unlike me, I'm usually quite willpowered.
I can't see how it would be good for working, the blurred vision of MDMA is definitely present, I usually have to put my face around 2 inches away from the computer screen to read lengthy paragraphs.
The comedown from repeated dosages is quite bad though. Just like the effects, it's a cross between an MDMA and a run-of-the-mill stim hangover, however not as bad as the hangover from MDMA and speed. Snorting over around a third of a gram will leave you with a blocked nose the next day as well.
Don't be fooled about the rumours that it's impossible to get a comedown off this stuff, especially when combined with sleep deprivation. The major downside is the jaw tension, I had devastating gurn injuries after my first half-gram session. I still love it though, possibly more than any other drug.