Because there is, oddly enough, a glut of banjo players around here I've been thinking it would be nice to get comfortable enough with my mandolin that I can take it along to jam sessions and at least be able to hack along with the I / IV / V chords (and occasional bVII) that make up most of the tunes in the event that 3 or 4 banjo players show up.
It's also partly a matter of strategy; someday I plan to have a go at the fiddle, and since the fingerings are the same any work I put in towards mandolin should give me a leg up when I eventually get my hands on a decent violin and some instruction.
Plus there's the 4-string, mandola-scale cigar box guitar that I'm currently building. The tuning is one fifth lower (CGDA) but the interval from string to string is still one fifth, so there's a lot of carry-over there, too. Once I finish the thing I want to be able to play it well enough to get something recorded on it!
I haven't gotten much from my few mandolin books that I didn't already have from years of plectrum guitar; they're very tab oriented and gloss over theory as it applies to the instrument, or scales.
Thinking back to my teenage guitar lessons, one of the most useful things my teacher ever did was to teach me the blues scale, and touch on the basics of improvisation. Most importantly, as you diddle around the scale make sure you hit the root note of each chord as it changes. He diagrammed out a closed-position, pentatonic blues scale for me and wrote the locations of the I, IV, and V root notes, and I spent quite a lot of time working on just that. I got pretty good at cheesy blues jamming with my friend, who was taking lessons from the same guy. (Thanks, Chris - If I could track you down via Google I'd drop you an email!)
So, I decided to start working on the same thing for mandolin, starting with Deep Ellum Blues (AKA Deep Elem Blues). It happened to be played at the People's Pint last Wednesday, and got stuck in my head. I think it was in D, so that was as good a place to start as any. I do love how logical mandolin is; look at the way everything repeats one octave higher on the 5th string of each subsequent fret! (And two octaves higher on the 10th fret of the next string, etc.) I imagine it's quite a bit easier to master the fretboard than guitar or banjo.
And of course, I'm not about to put my banjo down either... although the old-timey stuff seems to have pulled me away from the bluegrass/melodic practice I was threatening to do all summer. Playing with other people is addictive like that.