A good seamstress is best friends with her seam ripper. Not because she can't sew, but because she's probably a perfectionist.
Seam rippers, like other sharp tools, get blunt. Don't spend lots of money on fancy seam rippers, buy small inexpensive ones and change them often. You'll notice the difference. A sharp seam ripper will 'rip' seams quickly, efficiently, and cleanly. A blunt seam ripper will not.