I would agree with allround as they do everything you expect a slalom board to do quite well. I've had/have large, medium & small and for their sizes I think they are one of the easiest slalom boards to use- comfortable, go well through the chop and the flat, faster than you will ever need, nicely controlled and well constructed with the 07 onwards boards. Stick with the standard fin as it's not too bad and any further fins get something similar, if you don't get standard fin use something with a little bit more rake than say a select SL7. They have nicely rounded rails so it wont trip in gybe or chop yet they go great upwind also.
Downsides of them- some guys have had spinout issues. Start with footstraps in middle hole position, mast track about 1/3 from the back and don't go too small with the fin and you wont have a problem. It seems as though the airpipe/cutout make the board quite loose at low speeds. The airpipes add a bit of weight, don't bother using the cutout plates. To me they gybe great (i'm not a great gyber though) it was one of the things I noticed immediately when I first stepped on them, however they do have a tendency to have a slight slide when you put them onto the rail in the gybe but stay committed and they track round fine.
From all the slalom boards i've tried the SX's are probably the most balanced, they have an even volume distribution so you wont sink the nose like some other boards. They seem to be favoured by 90kg+ guys, if you have the power to drive them they just keep going faster. I am highly critical of boards, if they don't do something however minor I will move on from them pretty quick- i've still got an 07 medium and small and aren't parting with them anytime soon.
As for keeping similar shapes, boards don't have to change very much year to year once they've hit on the right formula and that is one of the things that has appealed to me with both the SX's and the isonics. Get it right then refine... but give them nice graphics