Somethin tells me the unfortunate unexplained angelfish death previous posters here have experienced was the result of cyanide trapping. The practice is all too common to try and stun fish with cyanide to make them easier to catch. Problem is, if too much cyanide is used (which is often the case) the fish suffers internal damage and dies shortly after transport/sale.
Dwarf angelfish are actually very hardy fish if you ask me. My old flame angel survived my 3.5 day power outage in which the tank had a mere 600gph worth of flow, no light, no heat, and temps dropped into the 50s... Came through just fine, no ich, no illness, no seeming stress once the tank was returned to normal. If you ask me, that's pretty hardy.
All dwarf angels can be moderately aggressive towards other fish in smaller tanks say 30-40 gallons. In larger tanks they're usually model citizens towards other fish. All dwarfs also have the ability to nip at corals, most often SPS, Open brains, or leathers, but I've heard tell of them nipping just about any coral they set their minds to. Some individuals will never harm a polyp in your tank, and others are destructive terrors... Unfortunately you never know until you get them in your tank at home. If you purchase one and plan to keep it with corals, make sure you're prepared to either break down the rock stack to try and catch the little bastage, or remove the coral it likes the taste of.