OK, my 2005 137 sx needs replacement.
I learnt on this board and I have been happy with it. I am keen to replace with another slingshot board. I also have the fuse and glide.
So I need a twin tip to use at my local which is flat water, for > 18 knots. I understand the sx provides a more comfortable ride, and the misfit and lunacy are for more aggressive riding: more pop, faster etc.
I appreciate the comfortable ride but I am also interested in a faster, more pop board. I can't make up my mind and I can't demo.
Appreciate any feedback from anyone that has tried these boards.
I can't comment on the SX, but have recently got myself a Lunacy and have also ridden on the misfit for a bit so I'll outline my own experience to try and help you decide.
The Lunacy is an awesome board, but frighteningly brutal. [}:)] It's hard on knees, the landings from big jumps can be really jarring, particularly in chop, and the wide ends send a lot of spray into your eyes if you lean too far forward. Having said that, the pop out of the board is something else altogether! just load up and go for it - as high as you want to go with the kite still nice and low. Landings from powered jumps are also made very easy and as long as you land board somewhat parallel to the water, it seems to stick! Great board for flat water and aggressive riding, not so good in chop, but still manages ok. The Lunacy performs brilliantly upwind, works really well as a light wind board and absolutely rules when you're way overpowered. The edging ability of this board allows you to edge the kite right forward in the window, cutting the power and controlling your speed. It also handles high speeds very comfortably and smoothly.... plus it looks awesome with the bright green base!! 
The misfit is like a comfy old couch, favourite pair of sneakers or tracky pants going from the Lunacy. You put it on and it's comfortable, predictable, direct and feels like an old favourite from the first second while still being heaps of fun! It's a familiar feeling to the Lunacy, but without the anger. It is an awesome board also and has heaps of pop compared to most boards I've tried (other than lunacy) and the landings from any height/speed are so soft and easy to control! I haven't ridden it enough to find any negatives with the board and from my experience I have really enjoyed riding it! 
They're very different boards, but you can tell same family. I have no affiliation with SS, and I just go for the gear that will suit me best for what I want to do. I'm just giving the big thumbs up for these boards cause they're both awesome and I would definitely say go for either of them, but really assess what sort of riding you do most of and whether you're better suited to a pure flat water machine (Lunacy) that will excite/thrill/scare you endlessly [}:)] or something that can handle a bit of chop and waves and be a bit more forgiving on the knees, yet still be a blast to ride!!
I'd definitely recommend doing a bit of a demo if you get a chance!
Good Luck.
Thanks a lot for the detailed response.
I'm over 40 and my knees to get a bit sore after a two hour session so it sounds as if the Misfit will suit me better.
Will try to get a demo.
yeh definitely not bad in chop at all, just hurts the joints if you cop an "uphill landing" on a wave or chop... can be very jarring.. still love it though 
Where are you located in vic? i have a lunacy you can give a crack... If we ever get wind!!!!
I find its awesome for hard landings, hence why i bought it. have not tried the others thou
I am in melbourne. Mainly go to Altona but venture down the coasts occassionally.
