Opinions please - is it worthwhile running an exit node on a home DSL
Nodes are nice to have around.
potential abuse from exit traffic more so than limited bandwidth. I've only
That's up to you. If you don't mind the odds of the queens best afp waking you up and borrowing all your stuff for a while till they figure out it probably wasn't you who posted that crap on the web... then you're fine.
Letting your ISP know you're running an exit would probably also help you there.
had it up for a few days and the bandwidth is being used.
That answers that part of your usage question, some people get no traffic thus do other things.
running as a relay?
Depends on you mostly. You could also be a bridge, obfsproxy, maintain the wiki, fix bugs, etc.
would make this node more useful, while not greatly increasing the risk of abuse reports coming my way?
Most abuse comes from http/s web cretins and sometimes filesharing. Though the infocalypse horsemen are always a threat. Specific authenticated and encrypted protocols like ssh, imaps, pop3s, submission, xmpp, and so on tend to be quiet.
Just read through the archives of this list, other answers are all there. Exit boilerplate and complaint templates, exonerator, and so on.