This card is probably “long-run worse” in most decks that play Signets than the appropriately-colored Signets, but even in those decks there are curve points where it is probably better. For example: on turn 2, Fellwar Stone can tap for a mana, which might be relevant to, say, a Sensei's Divining Top activation. Definetely a key player in Commander.

 

Conclusion: Most Players under-appreciate certain elements of design. I'm not certain this card should be played alongside or instead of Signets most of the time, only that overall, it is not played enough in decks like those. The contributions of Fellwar Stone are subtle, but strangely no different than in 1996, when some Big Blue decks figured they could piggyback White mana to activate their [stolen] Kjeldoran Outposts and others, um, didn't, passing cleanup with unspent Sky Diamonds on their boards.