I updated my firmware sometime ago on my DIR855 and have had nothing but troubles with it. Unfortunately the firmware will not let you flash backwards. Issues are plentiful but the main ones are wireless restarts and router reboots randomly and often.
I then started to notice many others were having the very same issues ( refer to http://forums.dlink.com/index.php?board=187.0 ) however the real surprise was that once I looked out side of my own product and into DLinks other Wireless N products, I found that the DIR655, DIR825, DGL4500 where all suffering the same issues based on recent firmware upgrades. ( Refer to http://forums.dlink.com/index.php?board=144.0 as an example )
DLink have released beta firmware on one of the products to help solve the router reboot issues with some mixed success ( Refer to http://forums.dlink.com/index.php?topic=8823.0 ) however the bad firmwares in question have not been pulled from their site and some have been up for more that 4-5 months, so potentially, the number of users affected is growing.
There is a site that is tracking the issues, causes and potential workarounds that you might want to have a look at, refer to http://dir855.wordpress.com/
This post is to make everyone aware of the perils with DLinks current firmware revisions and to perhaps hold off on any upgrades until DLink can prove their stability.