I use one similar to this and it's been a game changer. Super sharp and easy to open up seams. I find a good pair of tweezers or hemostats help in getting rid of the thread ends
I hate to tell you how many times I unpicked stuff in a star block I just finished (more than 10). However it made a huge difference having a decent seam ripper. The one I use has a razor edge and makes things go faster. It's very similar to this one, but mine is metal & has replacement blades. https://www.amazon.com/Havels-Surgical-Ripper-Serged-Regular/dp/B0018N9ATK/ref=asc_df_B0018N9ATK/?tag=hyprod-20&linkCode=df0&hvadid=167152383254&hvpos=1o1&hvnetw=g&hvrand=12697553521664814999&hvpone=&hvptwo=&hvqmt=&...
Seam rippers don't always fit. Its not because its cheaper or safer. In fact, its a pain to use. But some stitching is to tight for a seam ripper.
http://www.amazon.com/Havels-Surgical-Ripper-Serged-Regular/dp/B0018N9ATK