I have a negative reaction to ci[1] whenever I encounter it, and I am far from the only one. I don't think it's just because I'm a native English speaker, as I've heard the same thing from non-natives. Zamenhof himself advised against its use, except in poetry (as with using l' for la, which is also annoying when seen outside of poetry, for some of the same reasons).

You can use it if you want - far be it from me to tell anyone what they can and cannot say - but just be aware that it will color your communication negatively with a lot of people if you do.

So, to be courteous to your audience (unless you know for sure that they don't care, which you cannot in a public setting), I would advise against the use of either ci or l' unless you're writing a sonnet or limerick or something along those lines, where it won't strike people as obnoxious.

Plus, as far as Esperanto goes, I tend to accept any advice Zamenhof had to offer[2]. If he said 'don't use it', I'm good with that. But that's just me.

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[1] The only time I ever hear thou is if someone is reading a King James Bible out loud. I don't think anyone uses thou these days. Maybe the Amish; I'm not sure about that, though. I'm also not sure to what extent that colors my reaction to ci, however. Ci is like standing a weird distance away from someone while you're talking to them - it violates a social convention, and makes the other party uncomfortable. Thou is odd for its archaism, but not really offensive (for example, I won't get mad if an Amish person says thou to me; it might even be kind of cool, in a hey-you-don't-hear-that-every-day kind of way).

[2] It's less blind deference to authority, and more just an acknowledgement that although Zamenhof formally relinquished all ownership and control of the language forever, I think his thoughts concerning the language still deserve greater weight than do the opinions of others[3]. Call it fundamento-ism[4].

[3] Zamenhof would probably disagree with me. He just seems to have been that sort. Whatever.

[4] Although his admonition not to use ci is not in La Fundamento. So don't call it that. Or do. Either way.

I don't have so much of a negative reaction to it. If someone used it in conversation, I certainly wouldn't be offended, I'd just assume that they natively speak a language with the informal or singular 'you', and they were trying to be either specific or informal. But that's probably only happened twice. More often I find it in translations from languages in which it's more handy to distinguish between singular and plural 'you'. All in all, not very useful for EO-only discourse, understandable but a bit weird in translations.

I don't use ci myself, and it is true that it fell out of use very early. In the Fundamento we have (No. 16, second item) Ci skribas (anstataŭ „ci” oni uzas ordinare „vi”). It is mentioned in section 4, lesson 2 of the Linguaphone Esperanto course published around 1930, the only time that Linguaphone have produced Esperanto material; this was in the heyday of the language, between the two world wars of the 20th century.

Some people have tried to resurrect it now and then, usually without much success. Here's a past example in the Esperanto-speaking forum.

However I'm not sure why anyone should regard it as offensive. I have occasionally used 'thee', more light-heartedly than anything.

Not using "ci" in usual and normal conversation, doesn't mean that we can never use it.
"Ci" is in the "fundamento" not as an accident or an error, and so it must be some occasion where the use of "ci" is not only possible but preferable.
By the way, if I use "ci" with someone, everybody listening will quickly understand if my use is friendly intimate or offending!
That confusion is not possible in real life.

Is 'ci' offensive? Yes, when I found it in a translation I purchased of one of the Maigret series, I was tempted to write to the publishers to ask for my money back.

What the Sam Hill is wrong with you people? Let's call a spade a spade here: The use of "ci" in and of itself is not offensive; rather you take offense at the use of "ci"......NOT the same thing at all. That ci is offensive on its face is an absurdity. If an English speaking person who does not speak or has not learned any other language (alas, not a dying breed!) learned the word as "the singular you, used to one person" with no mention of T-V, I guarantee s/he would never consider being addressed as ci to be offensive.

Are you gonna be all offended when the next Quaker or Yorkie you come across calls you "thou"? Are you gonna be offended when I fall back on my former religious tradition and start addressing you individually as ci? Well, then, that's just your problem, not mine.

lemme ask thee a question, anyone else that takes offense at being addressed as ci: What, truly, is thy first reaction when thee's addressed as "ci"? Does thee think: "That rat b::::rd! How DARE he talk down to me!! I'm never talking to that arrogant toerag ever again!!" and stomp off in a huff? (hyperbole for emphasis on the stupidity of taking offense)

Or is it " 'Ci?' Hmm....I wonder why he uses that? I'll have to ask him."

Sorry, but mature, tolerant people react in the second way.

erinja:Almost no one uses "ci". I personally find it presumptuous and slightly offensive when someone insists on using it with me. .

Presumptuous and offensive? Seriously? Well, I find it presumptuous and offensive when people expect me to violate a religiously held precept (one that violates NOBODY'S rights, I might add), or even just a longstanding personal linguistic habit just because they don't like it.

sudanglo:Is 'ci' offensive? Yes, when I found it in a translation I purchased of one of the Maigret series, I was tempted to write to the publishers to ask for my money back.

Maybe they have just omitted the "ĉapelito" over "c". I can't think of any other reason they have put "ci" into translation.

Miland:I don't use ci myself, and it is true that it fell out of use very early.

AFAIK it never was used frequently - it certainly wasn't in the Unua Libro.

As for finding it offensive - in other languages it can certainly be somewhat offensive to use the informal pronoun in a situation where this is not expected. In Esperanto it simply cannot be determined - it's so rarely used that you cannot really tell what nuance it "normally" conveys.

Certainly if I see ci, I notice that that person talks in an exceptional way, and their intention may be to sound offensive - though normally it just sounds strange.

I don't see "ci" used anywhere, except maybe to approximate the English "thou". I think it's worth avoiding its use, simply because it is so infrequently seen that one might not immediately understand why the interlocutor is using it. I would not take offence at anyone who addressed me with "ci", although I would not employ it myself. I actually respect Esperantists who are not afraid to be a little creative and step outside the boundaries of a Euro-centric style.

as with using l' for la, which is also annoying when seen outside of poetry, for some of the same reasons

I guess I'm guilty of that. But is it really so wrong to occasionally condense "de la" to de l'" for euphony? I have one (very professionally-done) EO translation of a novel by Aleksej Tolstoj in my library that does this as a matter of course. I must admit I found it much more pleasing on the ear, without sounding inappropriately "poetic". "De la artikolo" is a mouthful compared to "De l' artikolo", for instance.