Loved the system in play. Used it in a physical challenge the other night:
Scene: The party has been alerted to an undead menace coming from the village cemetery; they decide to go to the cemetery during daylight when the undead are asleep and deal with it easily.
Problem: The cemetery is actually built on top of a steep, treacherous mountain which takes hours to climb.
Challenge: Climb the mountain quickly enough to make it to the cemetery before nightfall. Difficulty DC 19 (party's level 3)
Primary skills: Athletics, Endurance, Nature, Acrobatics.
Secondary skills: I allow pretty much any weird or creative use of a skill
as long as you keep succeeding at it. Once you fail even once at that secondary skill, you lost your chance can't use it any more.
Synergistic skills: I don't do the +2 for primary skills, but if one skill success logically facilitates the next one, that next one gets a +2 bonus.
(Nobody in the party thought to use Action Points, or didn't think it was worth it I guess)
Round 1
The
Wizard uses
Perception to try to find a shortcut in the bushes. He
fails, and the party sidetracks for a few hours to come to a dead end and have to backtrack. He can't search for any more shortcuts.
The
Warlock uses
History to see if she
knows of a shortcut or a better way to ascend. She also
fails, and the party has to backtrack again with zero progress. She doesn't know anything more about the mountain (can't use History any more).
The
Rogue uses
Acrobatics to determine the best path through the stone, which way to place your feet, where to find easy jumps, etc. He
succeeds, and the party is able to make some progress thanks to mimicking his path and his movements.
The
Paladin uses
Endurance (with +2 thanks to the Rogue's help) to continue push everyone along at a good rate. He
fails, and ends up getting tired and having to stop for a rest instead.
Round 2
Since there was more failure in the previous round than success, I introduce a complication: It begins to rain. I describe the water pouring down on the party and the path getting slippery and treacherous (no mechanical effects though, just flavor - though the situation is definitely more dire since a good chunk of the challenge has passed with only 1 success).
The
Rogue continues his
Acrobatics direction of the party and
succeeds again. The party is able to progress some more.
The
Warlock tries an
Endurance (with +2 thanks to the Rogue) and
fails, now she gets out of breath and has to stop.
The
Wizard keeps an eye out for any wild animals with a
Nature check. He
succeeds and warns the Rogue just in time for him to avoid stepping into a snakes nest, saving the party considerable time.
The
Paladin decides to pray to his god and ask him to stop the rain with a
Religion check.
He rolls a 1! 
Kord, the god of battle and thunder (which coincidentally is the deity that this Paladin worships) get angry at this display of weakness and smites him. With a lightning bolt. Paladin loses a Healing Surge and can't pray any more.
Round 3
Since the last round came out even in success-failure, no more complications, but time is running out, the sun is just starting to set, and it continues to rain. I describe the day getting slowly darker just as the summit of the mountain can be seen.
The
Rogue attempts to lead the rest of the way up with
Acrobatics, but
fails. He slips on a wet rock (by now very wet and slippery from the rain) and goes sprawling face first into the ground.
The party is now aware that they need to step it up, I make them aware of the Going for Broke rule and they decide to go for it.
The
Wizard looks for any kind of natural resource they could use with
Nature at +5 DC. We actually
makes it and finds some herbs that the party chews and gets a boost to their energy (something like a caffeine-leaf plant I guess, lol).
The
Warlock to run up the rest of the way with her newfound evergy (+2 bonus) using
Athletics at +5 DC and
fails. She did her best but it just wasn't enough.
The
Paladin makes a last desperate attempt at pushing his friends with an
Intimidate ("You better get moving or so help me I will kick your asses myself!"). He
fails it. His party is just too tired from the climb to listen any more.
Result: With 5 successes out of a needed 7 to get to the cemetery during the day, it was a partial success. I described them getting to the cemetery just as the last of sunlight winked out of the sky, and seeing skeletal hands slowly rise out of the ground. However, no undead were actually waiting for them and the boss (a Deathlock Wight) was still asleep, to wake up in a couple rounds (they didn't get to know this until later though).
I should note that even though there were many failure, there was never any feeling of the DC (19) being at fault. It was always reachable and it was 100% due to low rolling that we ended up like that.
As you can see, the round-by-round structure makes it easy to keep the scene interesting and fluid by knowing when to introduce new things, and keeps a sense of progress alive instead of devolving into a string of checks. I like that very much.
One thing I noticed is that the whole thing took A LOT less time to resolve than an average combat takes us. So no way I would give the same XP; I decided to be generous and gave 1/2 the XP that a normal encounter would give (300 XP) but I think 1/3 (200 XP) or even 1/4 (150 XP) would have been more in line.
Anyway, final verdict: We like!! Very much. The whole thing was easy to follow, stimulated engagement and interest, and got the players (players who have been through a BUNCH of skill challenges already) to start thinking up creative stuff again. No easy feat let me tell you, and this wasn't even the most interesting challenge in the world

. It just made the action align with everybody's 'narrative expectations' to a T, that I could tell.
I'm looking forward to tossing out some more of these tonight and if any of them turn out pareticularly interesting I'll be sure to post. Thanks Stalker0 for your hard work on it.