I have a very good english teacher who taught me to look carefully at editions and choose good ones. For translations, there are definately some translations better than others - really thin, cheap books tend to have worse translations. I usually use modern library classics, but rarely-I prefer to read in the original. Also, some editions have endnotes, etc...for example, the Oxford World's Classics editions have very good endnotes for books by Dumas, and a fairly good translation. There are also some very famous translators/very famous translations, such as the translation of the Diving Comedy by Allen Mendelbaum, which are considered the 'definitive' translation. Some editions also contain intros and bios of authors, which are usually good as they give context to the work. So it depends if you want just a good translation, annotation, context, whatever...is there a particular book you're trying to decide the edition for?