IF it is either Penguin, Modern Library, or Barnes and Noble, you can almost be sure that the quality of scholarship and if applicable, translation will be either mediocre, or the cheapest they can find. That being said, Penguin does have a wide range of texts, which makes it somewhat ideal.
Generally, it is the university press publishers that put out most of the good texts, from my experience, as well as lesser known labels. Norton certainly puts out some great editions, especially with earlier English works, such as Spenser and Thomas More.
Still, it is somewhat lamentable that one can not get a volume of Italian poetry with scholarship even beginning to approach the original - a 10 page introduction (which is probably the best one can get, most of which dealing with the translation and translator) doesn't really cut it - the standard Garzanti publications in Italian, for instance, are better annotated than some of the best editions of Shakespeare, something which I think the major publications in English, given the way classics function in English reading society, seem to lack.
Still, that being said, I would generally avoid most of these commercial publications for works you truly cannot live without, assuming you have the money to be picky. When it came down to it, I'm glad I have my 36$ Faerie Queene rather than the 12$ Penguin one, as the annotations are surely worth the difference in cost - though, when it comes down to it, I am fine with my Signet Shakespeare's Sonnets, finding the scholarship there good enough (though the Booth edition would be something I would love for my collection, as his notes are beyond comparison).
It all depends - generally for novels it doesn't matter as much, unless the setting is essential, and it is allusion and intertext heavy, like Ulysses (which I unfortunately have in an addition without any footnotes). As for poetry though, notes can make a big difference, especially in classical works, and definitely in 18th century works.
It really depends, though, I wouldn't buy a penguin edition of a removed translation - Chinese or Japanese, or something - simply because the quality of text is quite laughable (when translated for commercial purposes, the nuance and culture just seems to be appropriated into simple terms, as is the case in the Li Bai and Du Fu translation (rather primitively translated as Li Po and Tu Fu, despite the fact that Pinyin is the standard romanization around the world) - there is nothing there beyond a basic layer appropriation - nothing of the context, or of the poets themselves, which is necessary for reading their works with any approach at accuracy.
You would think, for instance, for 20$ a volume, you would get decent scholarship, but I think the Penguins are a tad price hiked, because of their wide range of availability, in terms of the fact that every major English-North-American-Big-Box bookstore seems to have a Penguin shelf.
Still, it is lamentable that we can't really get scholarly editions outside of scholarly presses - I would think that even general readers would enjoy a little bit more for their $
