I like Obsidian a lot and I check my own numbers against it to see if I am in the right ballpark. It doesn't cover all the cases I want to include, though. The fixed-length challenges don't allow for shorter or longer challenges. Also, the system doesn't work well for including a challenge within a combat.
Paul, I am curious if you have tried the combat section of my system in play? I created rules specifically to tailor Obsidian to a combat encounter, and if people find these rules insufficient, its certainly something I can look at more closely.