I figured out some speculative values the other day while bored at work - informed guesswork more than anything else, and only rough numbers. Counting only successes and challenges, the notions I came up with (we know a 50% success rate for the Ability Dice), so assuming a greater-than-50% success rate (but with chance of delay) from the cautious stance dice and a less-than-50% success rate (but also a chance of double success - a face with two hammers on, as we've already seen) for reckless stance dice, then the following trends seem to be true.
The basic ability dice have a 50% chance of a success each, so the chances of getting at least one success increase quite swiftly with multiple ability dice (the chance of failure is halved each time). The system would allow for essentially automatic success for a character rolling nothing but his ability dice... so it's fortunate that the system appears to require at least one challenge die for any task worth talking about (which by itself could negate one or two of those successes - we've seen both single-challenge and double-challenge faces on the challenge dice - and bring the odds down noticeably as more successes are needed to overcome the challenges rolled). Just from my own speculative ideas as to what each die could produce (which have already been proven wrong, but it seems appropriately illustrative), two challenge dice and a misfortune dice added to a roll of 1 cautious, 4 ability and 1 skill dice dropped the chances of success from almost 100% to a little below 50%, which seems instinctively about right.
Moving in cautious stance dice increases overall chance of success by a reasonable amount - which seems appropriate, given their purpose (you're being careful to try and succeed - low risk, low but consistent reward), but you might take too long with the task. For those familiar with WFRP2, Dark Heresy or Rogue Trader, it's like getting a bonus on a test, but having the test take longer and losing out on some of your degrees of success.
Moving in reckless stance dice (as far as I can tell and from my own speculation) overall chance of success in a similar manner, but the successes you do produce create greater rewards - faces with multiple successes seem to pop up a couple of times, meaning that while success for the reckless is more elusive, the rewards are potentially greater. Continuing the analogy from WFRP2/DH/RT, it's like taking a penalty but getting extra degrees of success when you do pass.
The skill dice seem entirely helpful, not only in increasing the chances of at least 1 success, but also in the chances of increasing the number rolled (the Righteous Success - the hammer with a + under it - allows an extra die of that type to be rolled, and we've only seen it on skill dice so far). The latter factor is an important consideration when you're rolling several challenge or misfortune dice, as multiple successes are valuable for overcoming the challenges rolled). The picture of them in the seminar video (near the beginning of part 5) shows that they have the Comet symbol as well.
Back to the Challenge Dice... there's a lot there. From the pictures in the latest designer diary, I can see 1 blank face, two Challenges (crossed swords), one Bane (skull), one Chaos Star, one 2x Bane (two skulls) and two 2x Challenge (two sets of crossed swords, which would negate two successes), which totals all eight sides. Overall per die, then, a 25% chance of negating 1 success, and a 25% chance of negating 2... a big deal, really.
The misfortune dice appear to be three blanks, two banes and a challenge - unpleasant, but nowhere near as detrimental as the challenge dice. If the fortune dice are anything like their unpleasant counterparts, then they'll have two boons and a success, so they're helpful but not as pivotal as Ability, Stance and Skill dice.
Obviously, without knowing the numbers, we can't go into any real detail, but I hope what I've pondered is helpful.