I like my clover seam ripper, but a lot of the time I use my small rotary cutter-maybe 28mm? and carefully pull the seam apart with one hand and just touch the threads with the cutter with the other hand and it is really quick to unsew. Have to be careful doing that, but I love how fast it is.
I don't have one of these, but it is an awesome seam ripper according to one of my friends...out of my budget...the flat blad sounds like what annel described.
http://www.amazon.com/Gingher-seam-r.../dp/B0026JSCGC