It's like other music - some of it is tuneless and inaccessible, some is very good. Mozart's Don Giovanni has been described as one of the three finest things God made, along with the Mona Lisa and the sea. At least half the arias are brilliant and the rest aren't bad. A big barrier is the language (let's forget the ENO for now, because the libretto rarely works in translation). Italian usually sounds best, French is OK, German is a Bit Odd although it hasn't stopped The Magic Flute being the most performed opera. Even if you spoke the language natively, you'd probably find it a bit old-fashioned. The stories aren't easy to follow in a foreign language so it's best to first watch a DVD with subtitles or at least get a scene by scene guide. Some of them have been filmed, like Joseph Losey's version of Don Giovanni; the catalogue aria, where Leporello is explaining to the already-distressed Donna Elvira how many women the Don has shagged (1003 in Spain alone) is rather well done.