There is no problem with Fastclick, if you are a serious publisher.
That said, have you thought about using Casale as a secondary agency instead? They are performing well at the moment.
In my experience, where I've used Adsense as a Default campaign on FastClick, the adsense results are very poor. It seems to me that is generally because for FastClick to show a default ad, they've already shown a dozen or so actual ads... and by that point, the viewer isn't likely to click any ad.
You may be better off using FastClick CPM campaigns as your backup for Adsense. ;-)