I've been watching his starts and monitoring his stats via FanGraphs and pitch f/x, and I've had many of my questions answered.  Just took about 5 minutes to do a very quick writeup of my impressions.

• His average fastball velocity (he throws mainly 4S and the very rare 2S) is 89.7mph.  He typically works, as we've been told, between 88mph and 91mph.  His fastball does have good movement and late life, and his long stride means he releases closer to home plate, so his fastball probably plays up a bit, maybe to a 55-60ish rating.  Nevertheless, he won't be confused for a power pitcher anytime soon.
• His present control (ability to throw strikes) is plus-plus.  He works efficiently and doesn't throw many balls.  He throws strikes with all of his pitches.
• His command is still erratic.  It goes through bouts of inconsistency where he misses his spots.  This may be partly due to the movement he gets on all of his pitches, but it's also probably an experience thing.
• That said, he has flashed the ability to go after the strikeout when he needs one, and work efficiently by throwing lots of strikes when the bases are empty.  I would rate his pitchability a plus.
• He throws a five-pitch mix: Four-seam fastball, cutter, slider, curve, and changeup.  He uses all of them regularly (over 10% of the time).
• His off-speed stuff is much better than we've been led to believe.  All of his offerings are major-league pitches and he gets whiffs on all of them.
• His cutter has been the most effective per FanGraphs and pitch f/x.  It comes in looking like his fastball (its average velocity is just a shade slower), but has more late movement, fooling hitters.
• His changeup and curveball still lack consistency, but both show plus potential.  He particularly loses the feel for his curve ball from time to time.  But he also throws some very good ones about 40, 50% of the time (to my subjective eye).  When he gets it right, it plummets out of the zone with very nice depth and late action.  His changeup also shows excellent late movement and a good 8mph speed differential from his fastball.
• His slider is probably his worst pitch, as it falls somewhere between a cut fastball and a true slider.  He uses it to get groundballs at times but it lacks the kind of depth where you could rate it more than average.  He commands it well, though, and it's definitely a major league offering as a result.

My overall impression of Lyles' ceiling has increased slightly since he was called up.  Previously I was viewing him as a good number three starter with a chance to be a no. 2 if all the chips fall in his favor.  His curveball and cutter are both better than I expected and I'm starting to view him as a guy who has a good chance to become a no. 2 starter in the future as he gains more experience and consistency.