Absolutely captivating. I adored the original Morse series and was happy enough to watch 'Lewis' continue on when it finished (even though I sorely missed the wonderful John Thaw) but Endeavour is in a class all it's own. Wonderful, world class casting and acting;
Roger Allam's Fred Thursday is, as someone else remarked, the epitome of the British senior policeman of the period, as the series unfolds you can see the character of Morse senior unfold, form and develop bit by wonderful bit. Shaun Evans is simply excellent as the younger Morse. Anton Lesser? Well, the actor who I first encountered playing Thomas More in the amazing 'Wolf Hall' is every bit as good as Chief Superintendent Reginald Bright. A character who comes over initially as frosty and rather pompous but as the series develops comes to display a completely different side. The 'nod' to John Thaw, if indeed that is what it was, in casting his daughter Abigail as the reporter 'Dorethea' was a master stroke. Both actors that play the younger Jim Strange and Dr Max DeBryn are superb also.
All in all I'd give the series a resounding 15 / 10 if I could. In a word, Amazing, and long may it continue.