While a 7-drop like this never saw 60-card Constructed play, it was clearly a pretty powerful effect for Commander right from the beginning. Being able to recycle Enchantments is quite powerful. You can discard something like Omniscience (which makes all of your cards have no mana cost to play) and bring it into play with one swing from the Sentinel. Being a trigger ability upon declaring an attack is particularly awesome, as the attack doesn't even need to deal damage to work.
There's another cool caveat to Silent Sentinel's ability. If you choose an Aura from the graveyard, you get to choose any legal target that it could attach to. However, since you aren't casting the Enchantment, the Aura technically doesn't target. This means that you can actually attach Auras to creatures with shroud (can't be targeted by spells or abilities) or opposing creatures with hexproof (can't be targeted by other player's spells or abilities).
The Sentinel's effect can also bring back the Theros Gods, as they are Enchantments, if they somehow end up in the graveyard. This gave it great synergy with the rest of the Theros block of sets that this card is from. However, the Bestow creatures that could also be alternately cast as Auras can only come back as creatures, since Bestow is an alternate casting cost.
The Commander who most often plays Silent Sentinel in their 99 is Daxos the Returned. This is primarily because the Sentinel was reprinted in the same Commander 2015 preconstructed deck as Daxos. But the Sentinel makes sense in that deck, since Daxos is all about the Enchantments! Other Commanders who have conscripted the Sentinel's services are Bruna, Light of Alabaster, Krond the Dawn-Clad, Uril, the Miststalker, Sigarda, Host of Herons, and Zur the Enchanter.
Silent Sentinel has proven to be a useful toolbox card for any Commander deck that uses a lot of Auras and/or Enchantments. Being able to dump a pricey Enchantment and get it back with a simple declaration of attack is a very powerful effect. While 7 mana is quite an investment, all you have to do is give your Sentinel shroud or hexproof and you should be able to get several activations of its ability, making it a fairly efficient way to recur your favorite Enchantments.