I bought this piece of kit originally just to play about with rough songwriting demos. This recorder though is good for much more than that.
As well as being able to overdub for song demos, you can set it to record MP3s at a number of compression levels. My band record all our gigs with it now so we can see where and how we need to improve. If you record whole gigs, you can examine not just the music but the chat to crowds in between numbers. Which moments of "inspiration" go down best and such like. The 1GB card can store around 4 gigs of 2 x 40 minutes sets at incredible quality levels.
It's also great for recording rehearsals, especially when you're working on new numbers. We circulate our efforts to all band members via email after rehearsal so we can so can see where to put our efforts next time around.
Not only that, but you can drop any MP3s into the Tascam, mute either the bass or guitar and play along in place of the muted part. Perfect for rehearsing alone - or just jamming along with your heroes. Oh, and as if that wasn't enough, it's got a built in drum machine for when songwriting inspiration strikes!