It’s really good, but not necessarily game-breakingly so.
It’s rare for a player character to get advantage to something in all circumstances like this, but “spells and other magical effects” is only a small subset of dangers a player character tends to face. It also only applies to those effects which would normally grant a saving throw.
The Sage Advice Compendium, in answering the question “Is the breath weapon of a dragon magical?”, suggests that the game’s designers intended “magical” to be more limited in scope than it may seem. For example, a red dragon’s fire breath is explicitly not a magical effect. “Magical” is defined as:
- Magic items, which monsters rarely use
- Spells, which PCs often use but monsters rarely do, and NPC spellcasters are typically a limited subset of enemies encountered
- Anything which specifically creates the effect of a spell
- A spell attack
- Fueled by the use of spell slots, which almost never occurs with monsters
- Otherwise explicitly declared “magical”
The majority of monster effects, even those which would clearly be considered supernatural, do not fall under this definition of “magical”. The Sage Advice Compendium document is considered to present official game rulings, though a broader understanding of “magical” as it relates to monster attacks would make this ability more powerful.
Magic Resistance would protect you from the aboleth’s Enslave ability, a planetar’s Innate Spellcasting, a basilisk’s Petrifying Gaze, a beholder’s Eye Rays, a death knight’s Hellfire Orb, a dryad’s Fey Charm, an empyrean’s Bolt (defined as a ranged spell attack), or any spell cast by an NPC spellcaster which allows a save.
However, it would not protect you against a red dragon’s breath weapon, an ankheg’s acid spray, a banshee’s wail, a behir’s Lightning Breath, a demilich’s Howl or Life Drain, a balor’s Fire Aura or Death Throes, a fire elemental’s flammable nature, a ghost’s Possession, a storm giant’s Lightning Strike, a night hag’s Nightmare Haunting (it’s magical, but allows no save), a hell hound’s Fire Breath, or any spell which does not allow a save.
It would also not protect you against melee, ranged, or natural weapon attacks, which are often the primary source of danger. Most monsters don’t rely on magical attacks for their primary effects (for example, the hell hound’s fire breath is only a recharge 5-6 power), and many monsters have no magical attacks at all.
In summary, magic resistance is really good when it applies, but in the hands of a player character it typically applies less frequently than you’d think.