What I can glean of Ruth from what she's told us of others

Ruth is great. By far the best protagonist we've followed so far, in my opinion.
This is a somewhat difficult position to hold given the remarkable lack of effort she has put into developing her own character. Sylvie and Lucille have both been thoroughly described, but Ruth's own appearance and character are of more mystery than than the frightful amalgam of physical and mental color that is Bernice; a character who inhabits like four pages. We almost never hear about Ruth's emotions or even opinions (beyond ways in which she chooses to describe things), especially not her individual thoughts (she does talk about the motivations of her and her sister as a pair fairly frequently, but not in particularly personal ways).
But this is part of what I enjoy about Ruth. Her apparent lack of self interest makes her seem chill and not self-absorbed (as I could imagine would be tempting when writing about ones own childhood), to a similarly charming effect as Sylvie's apathy towards social convention.
In fact, she shares a lot of Sylvie's more charming aspects.  They've both got a sort of go-with-the-flow attitude; Ruth rarely ever decides to do anything, and Sylvie seems not to have made a long term plan in her life (and very few short term plans). Unlike Lucille, the (in my opinion much less pleasant) foil to the Sylvie-Ruth mindset, Ruth doesn't mind Sylvie and even aspires to her way of life to some extent. Pages 105 and 106 present the reader with a rare moment of self reflection (Ruth is surprisingly self-aware given how little she seems to exist in her own head as opposed to drifting through the world around her) which is much more like what I would expect from a typical coming of age novel, during which Ruth details her fears about her "incomplete existence" and what not. During this little spiel she admits to suspecting that she and Sylvie are of a kind, and though she says she fears this possibility, it seems like she kind of wants to be claimed by Sylvie, and is somewhat taken a back when she is not.
I think the primary difference between the two characters, is that, while Sylvie's whimsical, childlike manner could make her come off as a little slow or oblivious (actually this is kind of ambiguous, but there are no real clues that Sylvie is especially aware of her eccentricity), Ruth's narration reveals a subtle but very sharp wit and surprising social acuity given her actions as a character. It's very possible that the wit we're reading is developed later in life, but regardless of which Ruth has it (old or young), it's certainly impressive and stands in stark contrast with Sylvie's assumed train of thought.
Ruth's subtle, but often really biting criticism of other characters illustrates a very well developed understanding of her world. It's almost Holden-esque, the way she undercuts their dignity with the subtleties of her descriptions, but I think she tends to do it in a much more clever way. Ruth's description of Bernice almost tells the reader more about Ruth than any passage that is actually about her (in fact, this is the line that convinced me of my love for Ruth). To say that Bernice "managed to look like a young woman with a ravaging disease" is a super concise and well worded commentary on the futility of trying to preserve one's vanity into old age. It's a smart thing to say, or at least I think so, and its hilarious to imagine this thought beginning to develop in the mind of Little-Ruth as Bernice gossips to their mother in the doorway.
Another delightfully mean description: "Fingerbone was never an impressive town. It was chastened by an outside landscape and extravagant weather, and chastened again by an awareness that the whole of human history has occurred elsewhere."(62)
The one legitimate criticism of Ruth that I can think of is that she's not very prodiuctive. However, I think that this book is very skeptical of traditional views on productivity. If Ruth can live her life happily with no ambition what so ever, I think that's totally fine.

Distortion from inside the Bell Jar

In class today, we talked about how Sylvia Plath's mother attributed The Bell Jar's less than flattering depiction of herself and other presumably realish people to an effort on Plath's part to document the distortion of truth experienced by people suffering from mental illness. Conflict of interest behind this statement aside, I think that it's interesting to think about how (and if) Esther's bell jar (so to speak) warped her (and by extension our own) perception of the Book's world.
We know that her judgement of other people can be a little harsh. She yells at Joan that she hates her when Joan's been nothing but nice. She also has a tendency to blame her own problems (and the problems that society imposes on her) on individuals. She blames the unpleasant, confining and autonomy-lacking nature of motherhood (and womanhood in general) on Buddy (calling him a hypocrite) who is not responsible for (though certainly benefits from, and perhaps perpetuates on a small scale) the system. She blames her career-related indecision and need to chose (a bad combination that will yield no figs at all) on Jay Cee, claiming that Jay Cee told something terrible to her when really all she did was ask about Esther's plans for the future.
But are these views a result of her mental illness? I can see a healthy (albeit somewhat frustrated) mind making these judgments. For one thing, they're not factual inaccuracies so much as odd, emotionally ridden opinions. For another, Jay Cee is an appropriate representation of the flaws in the limiting track to adulthood that she's following and Buddy is a good representation of sexist double standards; there is some legitimacy to Esther pinning these issues on them.
So is the bell jar warping her views of the world, or providing her insight into the true nature of things?
For the most part, during her decent into insanity (or rather the bell jar's decent onto her), the narrative remains strangely objective and, since the narrator has overcome her depression, there is theoretically some sort of system for potential filtration of straight-up factual inaccuracy (her older, sane self).
Even so, there are instances during which Esther is clearly perceiving things incorrectly. She claims that she can't read. Since she's obviously not illiterate this indicates some sort of issue between her brain and her eyes. Potentially even more troubling than this is that she keeps informing people that she can't read while obviously reading things. The headlines from her tabloids provide a significant part of the structure of the narrative during her illiteracy. She can read, she just can't read the intellectual things that she used to, which suggests a deeper problem. Similar to this is her supposed inability to sleep. She insists that she's not sleeping, but if she had actually gone for the number of nights that she claims with no sleep she would be dead.

"'I can't sleep. . .'
They interrupted me. 'But the nurse says you slept last night.' I looked around the crescent of fresh, strange faces.
'I can't read.' I raised my voice. 'I can't eat.' It occurred to me I'd been eating ravenously ever since I came to."
All of her complaints about her body failing her are ill-founded. I think that she perceives these physical issues because the bell jar is messing with her perception. Interestingly, it's actually messing with her perception of herself, not the outside world as the metaphor of a piece of glass between her and everything else would seem to suggest. Although, I suppose, in a sense, anything physical and removed from her conscious mind (like these problems seem to be) could be blocked out by the bell jar. 
Adding to the list of Esther's strange ideas under the influence of depression:
eating raw eggs and ground beef is acceptable, personal hygiene is too habitual to be worth it, slitting your wrists would be perfect if it didn't involve ruining your beautiful wrists. 

Esther's illness doesn't seem to manifest in ways as serious or concrete as straight up hallucination, but it certainly effects her opinions of others, normalcy and even herself.