Seconded. Flask is really awesome. The guy behind it (Armin Ronacher) is a beast of a programmer.
I've been developing with Django for the last two years (a screencast series by ericflo is actually what got me started), and I just recently started work on a major Flask project. Flask is terrific.
What Armin is doing with his curated extensions gallery is one of Flask's biggest strengths. You get all the awesome flexibility of a no-batteries-included architecture, but with a great set of fallback tools if you just want to get up and running fast.
Not to mention SQLAlchemy is just a ridiculously awesome piece of software, and it integrates with Flask very well.