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I find Buffer to be very helpful in letting me queue up posts for Twitter as I do my morning reading. It used to be that I'd do my daily morning reading, find something tweet-worthy, and post it then and there. That would often mean there were 5-10 tweets posted one after the other, at about 7-8am. Not the optimal tweet time.
Buffer allows me to "buffer" my tweets, grabbing them when I do my morning reading, but sending them out over the day. It's limited to 10 buffered items a day in the free version. A $10/mo "awesome" version will get you unlimited tweets. I haven't yet found a need to send that many daily tweets, so I'm sticking with the free version, for now at least.
How often used: Daily, almost every morning (except weekends, when I let myself off the tweeting hook).
Live without it factor: I could, because my daily "tweeting" activity isn't mission-critical. I'd just go back to spewing all my day's tweets at once. But I think this makes it nicer for everyone.