I've recently got into Goth/Symphonic metal and have bought a lot of compilations, most of them from Zyx, who seems to specialize in putting out such cds. Opera Metal 3 is one of the better ones. Like its two predecessors it contains a mix of symphonic, goth, and power metal, though it lacks the black metal bands (like Cradle of Filth) who appeared in volume 2. Like all such compilations it has its stronger and weaker tracks, but I at least found none of the tracks annoying and many of them quite enjoyable. I was even surprised by the 3 power metal tracks (not usually my favorite) as they are all pretty good.
For me the really impressive tracks are :Avantasia's Carry Me Over, Silverlane's Kingdom of Sand (both power metal tracks), Sirenia's The Mind Maelstrom, Battlelore's Exile the Daystar, Agathodaimon's Alone in the Dark, and Stream of Passion's When You Hurt Me Most.
While I'm a fan of Therion, Nightwish, and especially Epica, the tracks here are good, but not the best work from these bands.
Lunatica's Two Dreamers, and Cornatus's Mein Herz are two of the weaker tracks for me. They aren't really annoying, but simply uninteresting.
If you're looking for a compilation of this sort of music, I'd heartily recommend this Album.