STBs do wonders for stiffening up older body Subarus, and I'm sure that they would help accurately locate your suspension on the newer bodies somewhat, even though Subaru increased rigidity more than 100% from the GC8 to GDA redesign. If you just tool around on the street then don't bother, but if you drive the car hard they are an absolute must IMO.

I would say that STBs (and soon after, bushings) should come before suspension, sway bars, etc. Great shocks and stiff sway bars won't operate anywhere near their full potential if the frame that they are mounted allows flexing. I bought a Cusco rear floor brace for BF Legacy wagon off of Ebay for $35 ($165 MSRP), modified it to fit my BG wagon as a rear strut tower brace and it made an absolutely HUGE difference - before the brace the body would load up like a spring during a drift (I could even hear the adhesive on the rear window seals slightly seperating) and often cause me to spin when it sprung back, but now it sits super flat and even teeters on 2 wheels when I jack it up!!

If you are going to bother buying STBs, try to find some that have a preload option like the Primitive "Cam-Lock" bar and some Cusco styles. Although my bar was cheap, I had modified it so that I had to preload it even just to make it fit, which eliminates the latency in unsprung movement and activation of the bar. I think that a lot of people feel that STBs are useless because they don't install them properly by preloading them, and therefore waste the potential of the modification (I used a pole and a jack from strut tower to strut tower to preload my non-adjustable bar).

To sum up the long explaination: Yes they are absolutely worth the inexpensive price, but they are only worth anything if you install them properly.