Conditional Success and XP for Failures, what do you think?

Hi, I'm looking for some feedback.

In the design of the Lycadican game mechanics, back in the late 1990's I came up with the concept of degree of success (DOS) (For details of the basic Lycadican dice mechanics see this post.) Essentially the DOS is your modified roll subtracted from the target number. A positive DOS score indicates success while a negative DOS score indicates the degree of failure.

As I work on the Lycadican redesign one of the concepts I've read about in some other systems is the 'conditional success' and awarding XP for failures. While I'm not sold on the latter the former would seem to drive interesting game play and would be very easy for me to implement. I could essentially say that any DOS from 0-3 is a conditional success. i.e. 'You Succeed, but ..' some complication or trouble results.

Those of you who have experience playing with these kinds of rules, conditional success and XP for failures, what are your thoughts? How do they contribute to gameplay? Do they detract at all? What are some of the unintended consequences?

Thanks!