I think you made a good choice with the 1620. I burned a lot of crappy media with it and everything ended with a good burn. No coasters yet on DVD, 1 coaster on a Kodak CD-R (don't know if the problem was the image or the media).
It is very important to update your firmware, as everyone said. When I received my drive, it was B7H9, which is the 1st release... I was not able to read all of my DVD, but once upgraded, everything is fine.
Normally, you should not have problem with Fuji, TDK and Maxell medias.
May be you can try to update your burning software? I use the latest NERO, the latest DVD Decrypter and the latest Discjuggler and they all work perfectly with B7T9.