I would say that Fellwar Stone is a definite must for my 5-color decks and some of my 4-color decks. However, unless the deck deals in casting/playing my opponent's cards and paying for them, I don't usually put it in any 3 or less color decks.
Fellwar Stone and Exotic Orchard tend to be ignored by newer players, but experienced players know that they are amazing. Even in 2-color decks, Fellwar Stone is a staple. You have to remember that this is a multiplayer format, so chances are that the colors you are playing are being played somewhere else at the table. Also, lands like City of Brass, Forbidden Orchard, and Rupture Spire (for more casual players) are common, and any one of those lands gives Fellwar Stone access to all 5 colors.
In my meta, there are a lot of people who run more casual decks, and a few who run competitive. I, for one, have a Competitive Mono-Green
Selvala, Heart of the Wilds deck that consistently goes infinite in any number of ways turn 4-5. And along with myself, several players at my LGS play mono color and 2 color decks, so the Mana Fixing possibility is lower when playing in Pods.
I'm not trying to dispute that
Fellwar Stone has value, I just personally do not use them unless I can guarantee that Mana Fix, or my combos don't really care about the Mana it produces. That's just my personal play style and the Meta I play in.
P.S. I've been playing Magic for the past 22 years (23 this June).