Is Eucalyptus a good species for reforestation and/or agroforestry?

No actually its a terrible species for that, unless that is you live in Australia. I used to live in the north of Spain where its practically taking over and it has pushed out all the native species of tree out. The government has started a program of cutting down the eucaliptus and replaning native trees.

In Australia, yes. Especially if native to the area. Also depends on species.

For example. Eucalyptus cladocalyx (sugar gum) has been used in stock shelter belts in western Victoria for a couple of generations now because it has multiple uses - shelter, thinned for fire wood / fence posts, grows back even bushier for shelter when cut to stump and also for honey production.

A few years ago Victoria Forestry was also trialing Pinus radiata in Agroforestry plots.

Depends where you are growing it. It was a major factor in the 1991 fire disaster in Oakland, California, where once introduced it grew like, well, "wildfire" until eventually some actually caught fire and caused untold damage.

http://www.drj.com/drworld/content/w2_090.htm