The French idiom “mi figue, mi raisin” (literally: “half fig, half grape”) refers to someone or something that is neither entirely good, nor entirely bad. I guess the meaning of the expression can be rather well conveyed by translating it as “neither good nor bad”, “not entirely good or bad”, or “part good, part bad”, but… is there a common English idiom that express this idea?
A couple of rather-peripherally-related idioms are neither fish nor fowl and "some of this, some of that", neither of which is as close as Martin Beckett's Curate's egg suggestion, but both of which seem more related than phrases like "six of one, half a dozen of the other".
The idiom "mixed bag" meaning #3, "something tending to have both good and bad results or characteristics; something having a mixture of advantages and disadvantages" seems a good suggestion.
A somewhat similar idiom is double-edged sword
Currently defined in wiktionary:
(idiomatic) A benefit that is also a liability, or that carries some significant but non-obvious cost or risk.
Curate's egg is probably the closest thing in English - but it's bit old fashioned now. Nobody (except EL&U readers naturally) would know what you meant.
"Mixed bag" is when the item has aspects some good some bad. If it has one aspect in the mushy middle between good and bad you can call it "so-so".
Having looked in Petit Robert, I think "mi figue, mi raisin" has an element of personal demeanour about it which none of the suggestions above captures. "An ambiguous appearance of satisfaction and discontent, or serious and joking". I saw it used to describe the behaviour of Inspector Maigret (surly and mercurial by turns...).
I don't think there is a neat English equivalent... perhaps not least because, if I wanted to appear "mi figue, mi raisin", I'm not at all sure how I'd go about it. The closest I can imagine is "that way of saying something that is deadly serious, but almost dares you to take it lightly". Or vice versa. "Half serious, half mocking"?
As you can imagine, an idiom in one language doesn't necessarily translate literally into an idiom in another language. So the French idiom "half fig, half raisin" might be guessed to mean something like the English idiom "six of one, half a dozen of the other", in actual usage, it does not.
Checking with some French language enthusiasts (WordReference.com), the preponderance of answers from native French speakers suggests the French idiom is used to indicate the mood of a person. (I would ignore the English speakers, who may be correct but sometimes tend to Anglicize their thoughts.)
More concrete examples can be found elsewhere:
At linternaute.com it's defined to be a mood between agreeable and disagreeable.
Wiktionary.com states the the meaning taken today is along the lines of half willing, half unwilling; pleasant and unpleasant at the same time; to be of two minds (opposing) about something.
SensAgent defines it as that which satisfies and disappoints at the same time; to be serious and lighthearted at the same time.
Sources do refer to the Corinthian raisin commerce, and how figs sometimes made their way into the delivered products. The notion that raisins have a positive association and figs have a negative association may have come from this.
If you are looking for an English idiom with the same meaning, it would have to connote a mood in both negative and positive feelings.
If you are asked "How are you?" in French, then "Mi figue, mi raisin" can be used in place of "Comme ci, comme ça". An English idiom for this is simply "So so."
You can say you are of two minds if you want to express your feeling about a matter. This is better than so so at describing the mixture of both positive and negative mood.
Half joking is what you say when you want to be serious and lighthearted. An emphasis on the serious part would be something like "I was only half joking."
A more recent (1992, wikipedia) expression is meh, which can mean mediocre or unremarkable. It's the equivalent of a shrug, more like a grunt, but it has this meaning in the right context. An equivalent hand gesture would be the hand held out flat and rocked back and forth.
The phrase mixed bag can mean a mixture, but it doesn't necessary have to mean good and bad. (Please, have some jelly beans.) As for describing someone's mood (as with the current usage of mi figue, mi raisin), mixed bag could be used, but it's not common.
The same for the phrase six of one, half a dozen of the other. This idiom wouldn't usually be used to describe a mood, and doesn't have to describe conflicting concepts. (Do you want pastries or donuts today?)