Adj. Good, clever, well-conceived or -executed. Very commonly used in the U.K., generally to express rather less enthusiasm than in the U.S. (indeed, it is frequently noncommittal or ironic), and to refer to a an experience, quality, idea, or other intangible, as opposed to a person. The abbreviation “brill” has not penetrated to the U.S., as yet. “It was brilliant to program the Beethoven before the Carter.” (Anthony Tommasini, New York Times, December 9, 1998)/”I jumped on one of those little sleds,” [Philadelphia Phillies outfielder Raul] Ibanez said, “and went down a hill. It’s brilliant. Whoever came up with that – phenomenal.” (Philadelphia Inquirer, February 21, 2011)
notoneoffbritishisms.com 
I think this usage really became (deservedly) popular in this country when Bono used it (together with some other words 😉 during a Golden Globe Awards ceremony 8 or 9 years ago:
http://www.cbsnews.com/stories/2003/09/17/entertainment/main573729.shtml
No one has said brill in Britain since approximately 1990.
Not even ironically?
nobody in Britain has used irony since 1950 when satire caught fire
Everything the Brits say is with less enthusiasm than Americans.
My friends and I say “brill” as short-hand for “brilliant” all the time, and we’re born-and-bred Americans. I usually use it in the phrase “totes brill”, short for “totally brilliant”.
Nobody British has said ‘brill’ since the 80s. It’s VERY dated.
When I hear this word used on British telly (exported to America), I think it’s very usage is brilliant, and I wish I would remember to use it more often (than never) myself, when appropriate.
A friend of mine (once English, now Australian) sometimes says ‘Hah, blirriant’ in a fake Chinese accent. Not very PC, I’m afraid.
I think “brilliant” in British English now means something roughly equivalent to American “cool.” I’m not sure it has much of anything to do with mental acuity in this usage.
Brilliant in UK English is also often used sarcastically, as in: ‘My car’s broken down. Brilliant!’
Brilliant blog, by the way. [non-sarcastically]
I still hear Brits say “brilliant” all the time!
but never “brill”. Right?
In the UK, it would be perfectly appropriate, in the most formal writing, to use the word brilliant to refer to someone who is exceptionally clever or talented or to something or someone outstanding or impressive. For instance, “the brilliant physicist, Einstein” or “the brilliant lyrics of Bob Dylan”. In formal use I think it would only be used to describe a person or something derived from the intellect or ability of a person, such as an idea or song lyrics (so it is the originator of the idea or lyrics who is indirectly being described as brilliant).
Then there is the separate informal usage, which is to describe other things of which you wish to express approval, eg, “I had a brilliant holiday in Paris.” Also, the exclamation “Brilliant!”. This is well illustrated by a character from a popular British sketch show: http://www.youtube.com/watch?v=knYOcaQ-x5o
Both types of informal usage can be used ironically or sarcastically to express disapproval.
All these informal usages would be considered borderline illiterate if used in formal writing, such as in a serious newspaper article or business writing.
The formal usage is definitely still common. And I would have said that the informal usages are also in frequent use, although not quite as common as they used to be when the usage became fashionable for a while, probably around the same time as the sketch show in the video above.
Is it just the informal usage that is potentially a NOOB, or also the formal usage? I’d be surprised if the formal usage is not appropriate US English.
What you call the formal brilliant is definitely commonly used and accepted in the U.S. and has been for a long time.
I was under the impression that ‘brilliant’ came to England from Australia. Could be wrong, though.