For most automotive uses, an analog meter is fine. A digital meter, however, is VERY nice to have when diagnosing charging issues, because the difference between 12.7 and 12.9 volts can be difficult to read on an analog meter, but can mean the difference between a functioning and non-functioning charging system.
Curtis, the critical spec on a meter is the Ohms per Volt rating. 20K Ohm/V is about the minimum that is acceptable. Sometimes the ultra cheapo meters have a rating much less than that, and an ultra cheapo analog meter is more likely to have a lower rating than an ultra cheapo digital meter. This rating is the amount of "load" placed on a circuit when the meter reads the voltage. As for the voltage supplied, most analog meters that I've seen run on one battery, so that's about 1.5V. The digital meters usually have a separate setting for testing diodes, which will supply over the ~.7 V needed to overcome the junction of diodes and transistors and a setting below ~.7V for checking when you don't want to overcome the junction (handy when testing things without pulling them from the circuit). No decent meter is going to supply enough current to damage anything but some really exotic circuits, and I doubt that happens in all but extreme cases, and certainly not with anything we would be checking, including ECU's.
A $20 meter today is about all any of use will ever need, and that's what I use.
There's been a lot of threads lately on chasing electrons, and they have demonstrated a distinct lack of knowledge out here in GRM land. Perhaps a nice article covering the basics of electricity would be a good idea for an upcoming issue. In the mean time, google up the ARRL and see if you can DL a beginning electricity course.
