Shivan Reef is a premium dual land for any Izzet (UR) deck, and one of the most-played lands in the format for good reason: it enters untapped and fixes both your colors immediately. In a format where tempo and curve consistency matter, an untapped dual that always produces the color you need is worth the 1 life per tap.
The pain is real but negligible in most games — 40 life, and you'll often only ping yourself a handful of times before drawing into untapped duals or fetchable basics. The colorless option also lets you pay life-free when you just need generic mana.
Want it in: essentially every two-color UR deck — spellslinger, Niv-Mizzet, artifact/storm, control. It's a near-default include.
Skip it when: you're in a mono-color deck (you don't need the fixing and the life loss is pure downside), or in a heavy lifeloss/aggressive-aristocrats build where every point matters. Budget decks should still prioritize it — it's cheap and reliable.