There are a lot of good things going on in this MOC - very nice...
I'll start with what is probably the best part of this MOC - the wings. I really like the tiled border that the SNOT build made possible, and the piece you've used for the angled edges is perfect. The simple solution of hinges work well here, and I can't get over how spot on the detailing on the insides are. Just fantastic.
The wing pylons look great in spite of being studded and really captures the angular look of that area, something I've found difficult to properly execute at this scale.

As you said, the back sides aren't exactly the most presentable, but considering how little space you have to work with the wedge bricks are very effective in retaining the shape.
I love how well you've captured the squat, blocky look of the "tail" section and nailed the shape with those cheese slopes. What looks to be traces of a hinge in the back is not only accurate but a fun Easter Egg (BasOne


). The entire hyperdrive area looks really good; the shaping is somewhat off (can't be helped, I guess, due to the sloped bricks available) but the smooth SNOTed look of the region is quite fitting and the gaps are spot on.
Regarding the cockpit - the Lowell Sphere technique works very well here next to the angled wing pylons. The clips you've used to represent the laser cannons aren't too bad actually - they kind of do look like the ball-and-socket joints that are there after all (it's a shame they couldn't be placed higher up - or maybe the bottom just needs a 3x3 inverted dish to be complete... more on this later)
I'm a little surprised that you didn't SNOT the underside as well - limitations are limitations, I suppose...
Overall, a great MOC. There is just one suggestion I would make, though, and it's a pretty big one.
On MOCpages, Brian Tobin said:
One thing that stands out [on the TIE Advanced x1] is the cockpit "ball"...it looks big in comparison to the rest, doesn't it?...
...Well, it is, because the rest of the ship is smaller in proportion to it than LEGO's official version, every custom I've seen, and even every rendition ever sold in any form (except the new Master Replicas version, since they did the sensible thing...modeled it straight off the ILM original, not any other source!
This is really the only thing I would suggest reworking on this MOC. Since the x1 studio model used the same cockpit ball as that of a TIE/ln but was more compact, the cockpit ball ends up looking huge in comparison to the rest of the craft, as evident in shots like
this one. When I first saw this MOC, the shape of the main body reminded me of 10175, which suffers from the
same problem. This is also apparent when you consider the width of the "tail" of your MOC relative to the diameter of the cockpit ball in comparison to the same ratio on the
studio model. I think the issue may have arisen from the fact that you've represented the windshield with a 4-wide dish instead of a 6-wide dish. I'm afraid I can't offer much in the way of technique other than referring you to
Gareth's outstanding rendition (I've been wondering whether the ball could be done this way at minifigure scale but haven't had a chance to try it out), but I can tell you that a diameter of around 8 studs would better suit an MOC of this size.

Perhaps if you were to do something like
this...