Every time I hear someone trot out the phrase "I personally" it grates against my ears. I wouldn't mind so much, but it very commonly used by a wide variety of people. I grates most because I'm not sure anymore whether this is correct grammatically. Is it legal in American but not British English (or vice versa)? So, could someone solve this conundrum for me, finally, please?
It's a perfectly legal construction (although the word personally should probably be set off with commas), even if it is sometimes misused or overused.
As an evidential marker (something English doesn't require) it tells the listener that what you are saying comes from personal, as opposed to second-hand, experience. This is often left to context, or made more explicit with phrases like, "I saw it with my own eyes."
The second usage I am aware of is to distinguish between a simple choice made among equally valid options and an admonition. "I, personally, wouldn't do it that way, but this," is quite a bit different in tone from, "I wouldn't do it that way, but this." Where the first merely indicates a preference or habit of the speaker, the second carries an implied, "don't make me say, 'I told you so.'"
This is similar to many other prescriptivist rules: the large majority freely uses I, personally, but a certain minority doesn't like it. In the third edition of Fowler's Modern English usage, Burchfield recommends that personally should be restricted to two types of sentences.
The first is to signify that something is done by someone in person and not through an agent or deputy, if otherwise doubt could arise, as in the following example:
The Party Secretary personally warned me not to vote for the proposal.
In this sense it is often close to in person.
The second is to exclude considerations other than personal. I believe he is thinking of a distinction like this:
Though I personally dislike him, I believe he will be a decent Emperor. I will vote for him.
Here the speaker separates his personal, emotional perspective from a broader, objective one.
Burchfield calls the use of personally where it doesn't add anything to the meaning of the sentence "debatable" and "seemingly redundant", which are his euphemisms for "bad style". I believe many style guides agree, even though it is widely so used, as Burchfield also notes.
The safest choice, if you want to keep everyone happy, would be to use it where it adds something to the meaning of the sentence and abstain from it elsewhere.
It would be acceptable with commas:
I, personally, ...
Though, it is really a spoken form, resulting more often than not from false starts. If one were to write this, they should select:
Personally, I ...
Even then, there is some question about its necessity. It is reflexive and in most cases tautological. It would probably serve to add nothing more than emphasis to a sentence.
I think you should ask a doctor.
Personally, I think you should ask a doctor.
There is really no semantic difference between these two.
The short answer is that it is grammatically correct. Personally is an adverb, and in the example you reported, it is being used as adverb.
Examples of usage reported from the NOAD include:
She stayed to thank O'Brien personally.
He never forgave his father, holding him personally responsible for this betrayal.
They had made conclusions without getting to know me personally.
"Personally, I" and "I, personally," are sometimes very different.
"I, personally, first visited Argentina in 1992"
This might look like a redundant use of the word "personally" but I believe it means the speaker is saying they they have done something that others have done too. So someone might say "I first heard that album in September" and their friend could respond: "I, personally, didn't check it out till last week."
The latter is an indication of preference and also somewhat of an intensifier. "Personally, I'd rather we didn't go ahead with the merger."
The two can be used interchangeably in some contexts but in other contexts they mean different things.