I've built a couple of small Flask apps and I like it too. Django doesn't really seem to have a direct competitor in the same sense that Merb was for Rails though. Merb and Rails provided a similar developer experience but had some significantly different architectures. Things like Pylons and Turbogears are much more roll-your-own than Django and microframeworks like Flask are great for what they cover but usually app requirements will dictate Django vs Web.py, for example, and not developer preference.