The ST15EXP seems to be pretty stable to me, as I have yet to flip in it while on relatively flat river/lakes. I can still heel over on one side fairly easily to turn quicker. I'm 175 lbs, so there's plenty of "room" left over for gear.
Overall, I've heard the design recommended as a "great all around design", or a jack of all trades. I'd have to agree. Good stability, good cargo capacity, good speed, good maneuverability, etc. I'd definitely recommend this design to a new paddler.