Get yourself a bottle of Optiglanz Stainless Cleaner, it'll shine those downpipes up in minutes.
Be warned though, it's bloody evil stuff so use eye protection and 2 pairs of rubber gloves, and i also put a bin liner behind the exhaust to protect any paintwork.
It's not cheap, about £20 a bottle, but you only need to use small amounts so it should easily last a couple of years, and the results are very good in very little time.
Have a look at this:
http://foc-u.co.uk/index.php/topic,8580.0.html