I have had eight mainland Chinese teachers over the time I have spent studying, and I am still confused, because I find there are inconsistencies between my textbook's audio CDs, my teachers' explanations, and what I typically hear people say. I don't know how to read IPA so please bear with me.
In some audio CDs, and most of my teachers, they agree that "an" should be pronounced like the English "an" (e.g. an apple) and not like the English "un" (e.g. that was fun) and I do often hear it for cases such as "怎么办？" and "赶得到". They also tell me it's the same for 换 and 言 etc. (just not for 天 as "ian" is a different rule.)
However, the disagreement stems from others telling me it should be the English "un", and there are resources that also show this (I read through Pinyin tones pronunciation cheat sheet and listened to the accompanying links http://lost-theory.org/chinese/phonetics/ and http://chinesepod.com/tools/pronunciation, which follows this rule.)
I do also notice that some of the teachers in the first camp, while pronouncing 办 with the English "an", seem to pronounce 看 with the English "un". Not all of them, however. This makes things very confusing.
I have broken my pronunciation habits once and switched from the first camp to the second camp, and now am wondering if I should have done that, and if I should actually be switching back.
Can anybody clarify this?
(Note: The teachers that were staunchly in the second camp also pronounced "iu" as English "ew", not like the "iou" that it should be (就 etc.,) but I don't know if that's enough to disregard their arguments for what is considered correct pronunciation.)
UPDATE: Wiki article on pinyin claims:
an [än] an like British English "ban", but more central
Not ... entirely sure what "but more central" means, but going by the Macmillan Dictionary audio sample for ban and the CCTV link @Stan commented with it doesn't sound like the "an" in "an apple" nor does it sound like the "un" in "that was fun", but it sounds somewhere in between the two, what I'm thinking at the moment is that it sounds like the "an" in "hand".