Izzet Signet is premium mana ramp and fixing in any deck containing blue and red. Unlike most two-mana rocks, signets effectively cost zero net mana to activate (pay 1, get 2 back), so they ramp and fix simultaneously — turning one colorless source into two colored pips. That's huge for color-intensive spellslinger and combo decks.
Who wants it: Basically every Izzet, Grixis, Jeskai, or four/five-color deck. It's a staple in storm and combo builds (think Niv-Mizzet, Kess, Mizzix) where ramping into a high-pip payoff a turn early matters, and where filtering colorless mana into UU or RR fixes finicky costs. Also great in any deck running Sol Ring and friends to accelerate your gameplan.
When to skip it: Don't run it in mono-color decks (the off-color pip is wasted), and reconsider in pure aggro/landfall strategies that don't care about fixing. In heavy-removal pods, fragile artifact ramp can be a liability — but the upside almost always wins.
A near-auto-include for two-plus color decks.