I think it's a preference.
I sold my core set /FO and haven't missed them a bit. I just hate the name. TIE First Order. Sounds more like a squadron than a ship model. It doesn't fit the naming convention and it bothers me. What do you call em? Eyeballs and Squints is taken. Glares? **** I dunno, it's an eyeball, but it's a little different so...
So for me I mostly dislike them because of their fluff. I just don't feel 'em I guess. FOs... Eff-Ohs. Ugh.
But then there is the gameplay side of it and for all my life I just can't justify 3 points extra for a TIE fighter. So the /FO has a shield and a fancy move. I'm not swayed. And then you have to spend more points on it to get it into it's proper role. A TIE /ln just needs a warm body to do it's job.
Don't get me wrong, the /FO is great. I can respect that it's got some builds that put it in a competitive spot, and I realize that the /FO is an entirely different beast but it's one that just isn't for me. I looked at my collection and decided that if I was going to have a bunch of TIE's then I could really only afford to fly one kind, and I picked the classic ones, more based on personal taste than anything else. My gameplay concerns about the /FO are largely a reflection of my own play style.
But this way the point of the /FO I think. It was never intended to replace the /ln but rather provide players with another style of TIE build that doesn't invalidate or replace the older builds. I think the fact that some people still prefer the older TIE's shows that FFG has succeeded in creating a unique TIE variant.
