Depending on who you ask, they can technically mean different things (blinking is end-of-turn return, flicker is instant return) but they're often used in the same way so I'm just going to be bouncing between the two here and there
This is going in a choice few decks for sure, and is just in general a very good Green card
While this can technically wait until later in the process, the earlier you start the easier it will be overall
Aside from being a human, which is a very relevant creature type in Commander, Modern and basically every other MTG format, Esper Sentinel is good because it reads: Whenever an opponent casts their first noncreature spell each turn, draw a card unless that player pays X, where X is Esper Sentinels power. Its a bit like Rhystic Study, but the cost can be far higher if youve got things that make humans stronger or you can put +1/+1 counters onto Esper Sentinel
Their only redeeming feature in my eyes is that theyre a strong way of spending large amounts of mana in the late game