Some units do flex drills, most do not. I wouldn't count on it, but it seems like if you want that type of unit, you'll likely get it since most people don't have enough flexibility with their work life to do a flex drill unit. But generally, they're doing very cool stuff. If you're not in a flex drill unit, you will be expected to show up on regular drill weekend with your unit.
As for commitments, don't let the recruiting poster lead you to believe its only one weekend a month / 2 weeks a year. As an officer, you will be required to do a lot outside the drill weekend. I probably do about 2 hours of Navy related work a week outside drill weekend. I will also be completing my 5th AT in 2 years. Of course, they can't MAKE you do more AT's but once in, you quickly learn that this too is a competitive community and in order to make it through the ranks you need to put in more than the minimum. Its exhausting at times, but its still incredibly rewarding and is a great experience.