Indonesia in the wet
 

Thinking of delaying my trip to Asia by a couple of months, though this would mean travelling through Indonesia in September, October. Anyone been there that time of year and how will the roads be from Timor to Medan?

That is definitely during the rainy season although that just means it rains slightly more often than normal. Roads are pretty good in most places.

I've driven up the west coast of sumatra and didn't have any trouble with the roads. One place around Lahat was pretty bad but generally the roads are good. In south sumatra only travel in daylight though. Lots of bandits in the area.

I highly recommend Bengkulu and Bukkit Tinggi. Danau Toba was nice too.

We came through September - October heading east from Sumatra - East Timor. No problems with the weather other than it was bloody hot.
If you are travelling the other way through the island chain you will probably ride into the start of the monsoon - it hits the western islands first.
Roads are pretty good all the way through.