I'm with the Big Hitter on this - long, cheesy intros are a big turn-off.
What I look for are:
* Professionalism - good delivery, clear diction, no 'um' or 'ahh'
* Production quality - good lighting, in-focus, good sound, properly aligned camera angle, no background distractions.
* Factual - the good and bad, pro and con, clearly explained.
Phil Busardo has to be one of the best video reviewers out there - he ticks all the boxes. Scott (Igetcha69) is also very good indeed. GrimmGreen is knowledgeable but I his cheesy intros and distracting backgrounds (those posters - dude!) just grind my gears.
All in all 10-15 minutes is a good review but if it needs to be longer, and still keeps the entertainment level high, then so be it.