Once upon a time there was a discussion based on the premise that "very unique" is bad grammar. However, the problem starts with the mention of grammar. For some claim that it is not an issue of grammatical correctness but indeed semantics. Personally I would use a different label altogether, but hey, what do I know? Let me try to pull the various threads together and bang the nail in the coffin of this debate once and for all.

What we are really talking about here are gradable and non-gradable (sometimes referred to as extreme) adjectives. A gradable adjective is one which can be made less or more extreme by using a modifier, for example 'very'. Take 'pretty' as your gradable adjective. A flower can be 'quite pretty' or 'fairly pretty' but neither of these comes close to just straight 'pretty'. If we want to say the flower is more than just pretty, we can choose to say 'very' or 'really' pretty. If we are lucky, it could be 'incredibly' or even 'extremely' pretty. We are able to give the adjective, in this case 'pretty', different strengths or indeed grades - hence its grammatical tag of gradable adjective.

Are you still with me?

Ok, so let's try a non-gradable adjective. One of my personal favourites: 'superb'. Now, try combining that with very. Sounds wrong, doesn't it? Now try with really or truly. Ok, that works, in a way that accentuates the quality. But it doesn't work with extremely. Why not? Because superb is already of the very highest rank. Using something like utterly, truly, completely - any modifying adverb which suggests 100% is acceptable but anything less just sounds wrong.

Let me quote from Mr Swan:

Some adjectives and adverbs refer to qualities which are gradable - we can have more or less of them. For example, people can be more or less interesting or old; jobs can be more or less difficult; cars can go more or less fast. Other adjectives and adverbs refer to non-gradable qualities - we do not usually say that things are more or less perfect, impossible or dead.

So now is when I tell you that 'unique' is considered a non-gradable adjective. You can work out the rest for yourself, can't you? And if you can't, let me do it for you. Something is or isn't unique. There are no possible grades within that. In the same way that it cannot be 'slightly unique' neither can it be 'very unique'. However, the reason why we can use 'truly' with unique and it still sounds perfectly acceptable, is because we are emphasising the quality. 'Very' does not do the same thing; it quantifies and therefore cannot be applied to 'unique'.

One of the most important points that has been pointed out to a certain extent, is that English is a living language and this means that things which our great-grandparents said now sound stuffy and old-fashioned to us. Extrapolate that a little and it is not too difficult to see how things which were correct in Shakespeare's time are now deemed incorrect and vice versa. How do you know if it's right or wrong? Most of the time, provided you are a native speaker (or have lived in an English speaking country for some considerable time) you should absolutely trust your instincts. Does it sound right? Then it probably is. Does it sound wrong? Then it probably is.

I know that sounds kind of woolly and there are times when you want a hard and fast rule, but it does have a name (she suggests, helpfully). We call it 'collocation' and it just means the conventional combination of words, especially prolific in idiomatic and colloquial English. For example, and I borrow from my main man again, we say 'a golden opportunity' but never 'a golden chance' - there is no rule, here. It is just convention. It is what sounds right.

If it's important to you and you are in doubt there are many superb works of reference out there - use them. My personal favourite (indeed, I call it my bible) is Michael Swan's Practical English Usage, published by Oxford University Press. And yes, ok, I work for OUP, but that is partly in the hope that one day I will bump into my guru in the corridors and I will have the opportunity to prostrate myself at his feet ;)

Reference work used: (apart from years of teaching English and an MA in Language Variation and Change)
Practical English Usage, by Michael Swan.
Published by Oxford University Press.
Second edition, 1995
ISBN 0 19 431197 X