Percussion (bodhran, doumbec, doira, tar, riq, snare, various electronics) -- Pros : portable, I've been playing them since I was a kid and am pretty good, good dynamic range, they add a lot to the music.
Cons: I'm one of those annoying people who burst into rhythm at odd moments, drumming on my desk, on pencils, water bottles, copy machine, the office dog, my stomach, whatever is near my hands. Drum circles leave me cold. Many people think percussion mucks up music.

Piano, synthesizer -- Pros: Lots of versatility. Huge range. Melody, chords, harmony, and bass can all happen at once. Loud enough or quiet enough at need. I've been doing it so long that I pretty much don't have to think about how to make it do what I want.
Cons: Very hard to haul around. Not so great for the types of folk music I play.

Cittern/octave mandolin/Irish bouzouki/mandocello (call it what you will) -- Pros: Very versatile. I use it for Irish, Breton, Bulgarian, medieval, oldtime, rock, blues, pretty much anything works. Great sound (I play a Sobell). Unusual enough that the sound and the appearance is novel to a lot of people, causing them to actually pay attention. Same range as the guitar. I tune it EAEAE, giving me lots of open drone possibilities. I've also learned to play a full set of chords within the open tuning. I can capo up to the 15th fret, giving me a wide range of keys and registers. Easy to noodle on.
Cons: Can't think of any right now.

Guitar -- Pros: very useful in almost any situation and for almost any kind of music. Good dynamic range, good range of notes, bass, chord, and melody can all happen at once.
Cons: I'm not as good as I am on other instruments, so if I want to play anything difficult I have to figure it out in advance and learn it. Guitars are ubiquitous and so can get boring.

Tenor Guitar -- A recent addtion to the menagerie. I love the sound and the playability. I got it for Euro-trad music, but for some reason it has become my primary blues instrument. It works well with a slide.

Lap Dulcimer -- Pros: easy to play easy stuff on, easy to noodle on, tonality sounds great, the diatonic tuning imposes pleasingly spare harmonizations and melodies.
Cons: hard for me to play hard stuff on, limited range of tonality, the diatonic tuning imposes frustratingly spare harmonizations and melodies.

Celtic Harp -- Pros: Sweet. Good range. Can play chords, melodies, and harmonies all at once. People get all gooey over it. Good dynamic range. Lots of notes. Easy to improvise on.
Cons: A bit much to haul around. Hard to avoid sounding sacharine. Limited tonal range. Diatonic tuning imposes limitations; flipping levers can be a pain.

Banjo (five string, clawhammer) -- Pro: Love, love, love the sound of clawhammer banjo.
Con: I'm not very good at it. In fact, I'm pretty bad.

Oud -- I just keep telling myself not to fret . . .

John Peekstok