EMD runs about $400ish last I checked, so if you can hold out until a department will send you (and pay for you to sit there, as well), it might be a better investment.
My department sent me, as EMD is mandatory in our communications center. All medical calls must follow the cards; we do use all of the features (I could only imagine the firefighters' faces if I gave out "Rescue 1, you have a Thirteen-Delta-Two"), but it does cover our ass by giving as many pre-arrival instructions as possible, as well as determining if we need to start ALS.
If you are a Mass EMT, paying your own may is not a bad investment -- the 24 CEUs come in handy!!
-Mike