I have one (the Muse, with plastic optic). It's far from my most used lens, lol, but I do use it.
I never really look at a scene and say "you know, this would look good with the lensbaby". Instead, it's more like "I haven't used this lens in a while, I think I'll play around with it a little".
I have a couple other lenses that are like that too.
The effect is hit and miss - sometimes it's nice, other times it's not. You'll never know if you never use it...
I disagree that it can be done in PP though. Yeah, you can kinda-sorta do it, but it's not the same - and you can tell. Just like real soft focus vs. PP soft focus. You can get close, but it just 'feels' different. (BTW, out of focus is NOT soft focus...)
I don't think I would shoot once-in-a-lifetime events with a lensbaby, but it is fun to play with.
I've actually been thinking of getting the double glass optic for it - so I can actually get some sharp pictures with it. With the plastic optic, even if you're focused perfectly (which is hard enough at large apertures), it's still not 'sharp'.
The appeal is that it isn't perfect. Also that it's fun.
edit
There is a whole entire section of the forum dedicated to just Lensbabies, BTW...