It hardly matters what directions you want to go: bluegrass, country, old-time, celtic, gypsy swing, jazz, ethic styles, classical or baroque ... mandolin is an extraordinary capable instrument.
The mandolin is tuned symmetrically (in pure 5ths) making it especially amenable to melodies, very logical in improvisation, and capable of quick transpositions. Tuned the same as the violin (GDAE), it's first cousin to the fiddle.
Where ever you hear the violin or the fiddle, mandolin can easily go ... with few exceptions. Certain pieces are pretty much "unmandolinable".
Mandolin chords and percussion
Mandolin frequently serves as the snare drum of bluegrass music and "new acoustic" music. No instrument surpasses the mandolin's ability to contribute the combination of strong percussive hit plus a sweet, woody chordal accompaniment.
In the early 1900s the tenor banjo was popular due to it's volume and chordal versatility, but there were two issues with the tenor banjo: 1) Its scale length required "cello" fingerings. 2) It's tone was metallic and strident.
Since the advent of arch-top f-hole mandolins in the 1920s we have a much sweeter was of delivering high pitched chords.
Pitched roughly an octave higher than guitar, the mandolin can't match the deep, rich, long-ringing chords of a guitar accompaniment, leaving the guitar as the favored instrument for vocal accompaniment. Still many artists like Tim O'Brien use it effectively as the sole back-up for vocals.
Mandolin accompaniment provides a perfect rhythmic compliment to guitar; the two instruments work wonderfully together in many styles of music.
Mandolin as a second instrument
Mandolin is a great second instrument. If you play violin or fiddle, your left hand already knows what to do. You'll just need to make some minor adjustments to your left hand technique, learn chords, and learn how to use a pick.
Sometimes violinists get home late and want to play, sometime they just don't want to hear 70 decibels right under their ear, and want to play, the violin is sure to wake other household members. But with the mandolin one can whisper away musically into the wee hours, and no one's the wiser.
If you already play guitar, then you're probably pretty comfortable with the pick. You'll need to learn new chord positions, scale fingerings. If you read guitar music you'll find that treble clef notation applies differently to the mandolin.
Mandolin as an introduction to violin & fiddle
If you're interested in playing the violin or fiddle, but aren't ready to take the plunge, try the mandolin.The left hand logic applies directly to fiddle or violin or. Then you'd just need to learn how to hold the violin and the get well acquainted with the intricacies of the bow.