What a lot of the round ones need to do is add an anti-roll ring or equivalent on the end. I have Exacto knives that have an octagonal piece on the end that keeps it from rolling.
It's real hard to get a seam ripper you really like and have it roll off a table and under who knows what when you are sewing. (I've also buried the flat ones under piles of junk and the ones that retract, do break)
Here's a pic of an Exacto no roll knife with a comfort handle:
http://www.amazon.com/gp/product/B00...=1&*entries*=0
I'd pay that for a decent seam ripper that stayed put.