Lensbaby made a pretty good version--their "soft focus optic"---they're about $50 on ebay. The rendition beats the Thambar, I think. It's interesting that Leica's sample pix showcase some of the worst effects you can get with this type of lens if you haven't shot one before. If you want to spend a bit more for a fancy mount, there's the Lensbaby Velvet 56.
No, the effect is not the same as a soft focus filter. The thing that draws people to real soft focus lenses is the increased depth of field at larger openings.