Cuddy:
Thanks for that info, glad I didn't pull the trigger on that already.
I had a quick read through the user reviews on the BT one and some of them mention that it can't do "monitor all". I'm sure that any OBD device that I buy will probably end supplying more information than I will ever need but is worth going for one that does have that feature? The $29.99 one doesn't have any user comments on it, do you think this might have it?
I really don't know for sure, but most cars have their own manufacturer-specific codes which can't easily be read by generic readers. However, this uses a pretty standard ELM327 chipset which, from the limited amount I have read, should offer a decent level of functionality for the price.
Sorry I can't be any more specific.