Have you checked out PowerPivot? I've been developing in Excel for 15 years and created many a convoluted formula along the way. Two years ago I moved all Excel development to Powerpivot and I hardly write any formulas in "regular" Excel anymore. PowerPivot provides more of a formula IDE (along with DAX Studio), along with a ton of other functionality. I think it's clear that's the direction sophisticated Excel development is going.