If the disc allows you to reach distances you haven't reached before routinely - how can it be bad?
if a disc does not teach the fundamentals needed to throw better (better != just lower scores), then it doesn't do much for you if your goal is to become a better player in the long run.
i see certain discs as being like hybrid iron/woods in golf. while people may find these easier to succeed with, it is not an excuse for being weak with your irons. but this is frisbee. the first 600' throw was performed in 1987 with a lightning #2 hookshot. it shouldn't take a re-invention of the wheel to have distance that is sufficient for 85% of holes on 95% of courses.
also, accuracy and predictability is (nearly) always greater than distance (unless you are throwing say, over a river). if i had to play a round with 2 discs, 1 disc being a putter and my choice for the 2nd disc, it would either be a dx roc or a d cyclone, depending upon the course length and relative tightness of fairways, with the roc being the choice for shorter courses and tighter courses. that being said, distance really takes a back seat.
that being said, imo, any players that aren't desperate to add D (throwing 320+) will flip the roadrunner and the rr will hold a turn without fading out, making it a great turnover driver. however, for players that still have D that is easily attainable (D under 350' assuming a moderately healthy male age 16-40) i do not believe a re-invention of the wheel is necessary to obtain that either. if players work their way up to 350' with slower discs, learning how to hit lines, make discs work, etc. then hopping up to the latest and greatest will almost ensure 400' of D whereas skipping that step does not.
this may be a purist attitude speaking, but in disc golf (and ball golf where applicable), long term success is best achieved in the ways it always has been: lots of hyzers, tight approach game, course management, hitting fairways, and solid putting = wins.
similarly, in football, while people may buy into the run & shoot, west coast offenses, single back sets, cover 2 defenses, ultra-athletic qb's, marquee receivers, etc., it is solid swarming defense and pounding the ball on the ground that builds dynasties. if your running backs always get 3.5 yards per carry, you are unstoppable. if your defense always holds opposing running backs to less than 3.33 yards per carry, you are golden.
As these disc "break-in" what will it start doing - so I know what to look for.
it will turn and hold a slow turn that pans from left to right and falls out of the air at the farthest right point when thrown flat.
And if I like what a RoadRunner does before it breaks bad on me - what disc mimics the RoadRunner when the RoadRunner is in its sweet spot?
first answer is... since the roadrunner and leopard don't really mimic 99% of golf discs, i wouldn't suggest falling in love with either of them for straight drives. for turnovers, they are excellent.
second answer is the TL is probably the disc that is closest in its straightness but still has something that resembles predictable fade, especially in star plastic (i can easily say this is my favorite star disc so far).
btw, if you did get a freakish RR that is true stable/overstable, i wouldn't count on them staying that way.