I have used SCs for a long time, both the core systems in 4e and my own system.
Generally what I found over time was....less is more. I think SCs can be very useful, but it is VERY EASY to shoehorn them in to narratives where they don't belong. More often than not I've found, when in doubt....don't use a SC. A badly done SC is boring as mud, and feels really gamey and arbitrary.
The secret of a good SC is to craft it in a way that the players don't really know they are in a SC, and that usually takes planning and work. I don't think casual SCs are that great.
I also think the best skill challenges involve some sort of real "progression". If your scene is static, its often best to just make a few rolls and be done with it. SCs work better in things like chase scenes where the action keeps moving, prison breaks where you are moving from one area of the prison to another. A diplomatic skill challenge is best in a large party settings where you are moving from one NPC to another learning bits of info, rather than just sitting down with the duke and throw down Persuasion checks until you hit your magic number. Ie there needs to be a narrative reason to roll more checks. When you are in one place with one person and you are rolling checks just to fill your SC meter....it feels arbitrary and gamey. When you are making a new check because the chase has taken you to a new area....well that just feels narratively appropriate.