How is the pinyin 'iu' pronounced? How does one pronounce the 'iu' sound (as in liu4 = six)?

I have always said it and heard it to end in an [joʊ] sound, as in 'low', 'go', and 'sew'.

But once I was told that it should actually be a 'u' sound, perhaps something like [iu:] as in 'few'. (And from the pinyin, I suppose it would make more sense.)

Is a 'u' sound the proper way to pronounce it? Or is this merely a regional difference in pronunciation (where both are acceptable)?
The short version: I think English &quot;go&quot; is closer to the correct sound than English &quot;few&quot;. If you want a non-scientific description which is reasonably accurate, I would use &quot;low&quot; (let's switch to the same consonant) with an added but not emphasised [i] between &quot;l&quot; and &quot;ow&quot;.
The slightly longer version: Pinyin -iu is short for -iou, which means that if you stress the syllable and pronounce it very clearly, it rhymes with Pinyin you. For example, listen to the liu (select the third tone, it's longest and clearest) pronounced here.
However, note that the  diacritic (tone mark) in liù is placed on the u and not the i, which means that u is emphasised and i reduced. This can be written in several ways, but in Duanmu's The Phonology of Standard Chinese (2007), the broad transcription is [liəu] but in the narrow transcription, the [i] has been reduced to a superscript &quot;j&quot;, so [lʲəu]. This reduction is fairly obvious if you listen to natural, connected speech. It's also common to further reduce the [əu]. If you listen to the same sources as above, but select the fourth tone for the same syllable liu, you will hear the difference.
If you're curious about other learner-related problems with Pinyin, such as other cases where one letter stands for many sounds (i is the most obvious example because the sounds are completely different), I've summarised the important ones here: A guide to Pinyin traps and pitfalls: Learn Mandarin pronunciation, which also contains some audio samples.