I have a Lensbaby 2.0 Muse in Nikon mount with the doublet glass optic and the soft-focus flat-field optic. I also eventually got the closeup kit, tele extender and wide angle adapter. Sometime soon, I suspect that I'll get the new 80mm f/2.8 flat-field optic.
I quite enjoy using mine; I find that the restrictions and modifications that it provides represent a useful framework within which to work, some resistance against which to struggle. It's more like a Stratocaster than a Les Paul. It doesn't suit every subject, of course. The surface pseudo-profundity ("ooh, shallow DOF! exaggerated blur and subject isolation!") of the "sweet-spot" effect is itself a challenge, daring the photographer to go beyond the obvious and clichéd and find a way to use the effect to legitimately enhance the photo. I find this to cause me to spend more time looking at aspects of my imagery that I otherwise mightn't, and honestly some of my favorite photos have been taken with the Lensbaby. I'm sure that's just because I'm a lousy photographer, so don't get excited.
I also like the organic feeling of the Muse's accordion/bellows focusing. There's an immediacy to it that I like. The doublet optic is not a replacement for a 50mm f/1.8 Nikkor in any respect. It's not meant to be. The spherical aberration off-axis is deliberate and heavy. You can control it with aperture. It's not subtle. If you're thinking it's a poor man's T/S system, keep looking. This is an effect, a deliberate and controlled distortion. Some folks hate it. The BBC engineers who first tried recording Hendrix spent hours trying to get rid of the distortion in his guitar. Some folks hate Hendrix. Me, I like my Lensbaby. Probably means I'm an unimaginative photographer who can't differentiate between gimmickry and meaning. I should just use Instagram on my iPhone and leave the real photographers alone.
As for gimbal vs. bellows, it's simple: tripod or not? The Muse (bellows) is useless on a tripod. It has no purpose other than handheld. The ball version is probably somewhat usable handheld (never tried one), but is essential for tripod use. I love using my Muse for close work - I've been known to put it on an inch or so of extension tube and get some really nice results. Of course, it's just a gimmick and "nice" as far as my work goes is about equivalent to the first-culled crap from a real photographer. I suggest you avoid the LensBaby like a Day-Glo Holga TLR. Save your money, get a Leica. They have very little distortion, and what they do have is really really special and actually does improve your photographs.