Hi,
I picked up the mandolin after years of playing guitar. When I started on the mandolin, I thought of it as a small guitar and used it mainly as a rhythm instrumnet. Strumming along with a guitar is fine and gets the job done, but the mandolin was easily drowned out and, frankly, wasn't too interesting.
I started treating the mandolin more as another voice in the music, sort of like a fiddle or a tin whistle, but with easier chording. I now try (and I haven't mastered much yet) to strum some basic two-finger chords and use fills and runs. Irish-style mandolin can be wonderfully expressive. You can get a clear, crisp sound from the high notes. Recently, I've started messing around with the GDGD tuning. I'm liking the drone tones I'm getting and my left hand feels freer to pick out some neat fills and runs.
I haven't found a great tone from my mandolin when it comes to accompanying a singer without a guitar also present. I think perhaps the notes are too staccato, where a guitar might resonate longer. Of course, the problem could exist somewhere between the mandolin and the strap, if you know what I mean.
I gotta try those neutral chords listed above!
Rich