Allison Bailes' latest blog revisits flex duct. Check it out: http://www.energyvanguard.com/blog/joy-flex
Here's a comment I posted in the blog: I'd rather see my clients use flex-done-right than 100% hard pipe given the cost difference. The problem is finding contractors who build flex to impress. The most important criteria: use metal elbows where a gradual bend won't suffice, fully stretch the runs, and use an ample qty of wide non-metalic support straps so as to avoid sags and pinches. In the context of a $10k+ job, the additional time required adds a small percentage to what a botched flex job would cost. It all boils down to training, defined objectives (leakage, static pressure, and hopefully air balance) and rigorous QA.