Jane Q. Public <email@example.com>This is clean, clear, and unambiguous.
However, some tools (gpg, enigmail among others) ask the user to provide a "Comment:" field when they are choosing a new User ID (e.g. when making a new key). These UI prompts are evil. The savvy user knows to avoid entering anything in this field, so that they can end up with a User ID like the one above. The user who provides something here (perhaps even something inconsequential like "I like strawberries", due to not being sure what should go in this little box) will instead end up with a User ID like:
Jane Q. Public (I like strawberries) <firstname.lastname@example.org>This is bad. This means that Jane is asking the people who certify her key+userid to certify whether she actually likes strawberries (how could they know? what if she changes her mind? should they revoke their certifications?) and anywhere that she is referred to by name will include this mention of strawberries. This is not Jane's identity, and it doesn't belong in an OpenPGP User ID packet.
Furthermore, since User IDs are atomic, if Jane wants to change the comment field (but leave her name and e-mail address the same), she will instead need to create a new User ID, publish it, get everyone who has certified her old key+userid to certify the key+newuserid, and then revoke the old one.
It is difficult already to help people understand and participate in the certification network that forms that backbone of OpenPGP's so-called "web of trust". These bogus comment fields make an already-difficult task harder. And all because of strawberries!
Tools like enigmail and gpg should not expose the "Comment:" field to users who are generating keys or choosing new User IDs. If they feel it absolutely must be present for some weird corner case that 0.1% of their users will have, they could require that the user enters some sort of "expert mode" before prompting the user to do something that is likely to be a mistake.
There is almost no legitimate reason for anyone to use this field. Let's go through some examples of this people use, taken from some examples i have lying around (identifying marks have been changed to protect the innocent who were duped by this bad UI choice, but you can probably find them on the public keyserver network if you want to hunt around):
John Q. Public (Debian) <email@example.com>We know you're with debian already from the @debian.org address. If this is in contrast to your other address (firstname.lastname@example.org) so that people know where to send you debian-related e-mail, this is still not necessary.
Lest you think i'm just calling out debian developers, people with @ubuntu.com addresses and (Ubuntu) comments (as well as @example.edu addresses and (Example University) comments and @example.com addresses and (Example Corp) comments) are out there too.
John Q. Public (Johnny) <email@example.com> John Q. Public (wackydude) <firstname.lastname@example.org>Again, the information these comments are providing offers no clear disambiguation from the info already contained in the name and e-mail address, and just muddies the water about what the people who certify this identity should actually be trying to verify before they make their certification.
John Q. Public (Work) <email@example.com>if John's correspondents know that he works for Example Corp, then "Work" isn't helpful to them, because they already know this as the address that they're writing to him with. If they don't know that, then they probably aren't writing to him at work, so they don't need this comment either. The same problem appears (for example) with literal comments of (School) next to their @example.edu address.
John Q. Public (This is my second key) <firstname.lastname@example.org> John Q. Public (This is my primary key) <email@example.com> John Q. Public (No wait really use this one) <firstname.lastname@example.org>OpenPGP is confusing, and it can be tricky to get it right. We all know :) This is still not part of John's identity. If you want to designate a key as your preferred key, keep it up-to-date, get people to certify it, and revoke or expire your old keys. People who care can look at the timestamps on your keys and tell which ones are the most recent ones. You do have a revocation certificate for your key handy just in case you lose it, right?
John Q. Public (Old key, do not use) <email@example.com> John Q. Public (Please only use this through September 2004) <firstname.lastname@example.org>This kind of sentiment is better expressed by revoking the key in question or setting an expiration time on the key or User ID self-sig directly. This sentiment is not part of John's identity, and shouldn't be included as though it were.
John Q. Public (none) <email@example.com>sigh. This is clearly someone getting mixed up by the UI.
John Q. Public (3092 bits of RSA) <firstname.lastname@example.org>This comment refers to the strength of the key material, or the algorithms preferred by the user. Since the User ID is associated with the key material already, people who care about this information can get it from the key directly. This is also not part of the user's actual identity.
John Q. Public (no comment) <email@example.com>This is actually not uncommon (some keyservers reply "too many matches!"). It shows that the user is witty and can think on their feet (at least once), but it is still not part of the user's identity.
John Q. Public (offline long-term identity key) <firstname.lastname@example.org>Again, this is not a part of the user's identity -- it's a statement about how the user manages their key. If a third party wants to certify this, presumably they need to verify it. How would they decide whether this is true or not? It seems like it would make it more difficult to get certifications from people who are intent on actually verifying the User ID.
It's also (potentially) wrong: An OpenPGP User ID is bound directly to the primary key, but it is implicitly also bound to any subkeys that are associated with the primary key. if the subkeys are online, but the primary key is offline, is this comment correct? What if the primary key later adds a subkey that is not offline or long-term (e.g. they want to do frequent key rollover for an online subkey)? Do they need to revoke their User ID for this?
And where it's not wrong, it's redundant. An OpenPGP certificate is by definition a statement about long-term identity that is bound to a key in the first place. Why include any of this extra verbiage, which is only likely to add to the confusion?
I'm sure that such cases exist. I've even seen one or two of them. The fact that one or two cases exist does not excuse the fact that that overwhelming majority of these comments in OpenPGP User IDs are a mistake, caused only by bad UI design that prompts people to put something (anything!) in the empty box (or on the command prompt, depending on your preference).
And this mistake is one of the thousand papercuts that inhibits the robust growth of the OpenPGP certification network that some people call the "web of trust". Let's avoid them so we can focus on the other 999 papercuts.
Please don't use comments in your OpenPGP User ID. And if you make a user interface for OpenPGP that prompts the user to decide on a new User ID, please don't include a prompt for "Comment" unless the user has already certified that they are really and truly a special special snowflake.
UPDATED 2015-02-11: added "descriptions of key use" example, since that appears to be more common.