I have just signed up for TSA Pre-Check, but am having second thoughts about the advantages. I almost always travel first class (no, I'm not paying for that myself!) and thus have access to the first class security lines. If I use TSA Pre-Check, do I still enter security through the first class line? Conversely, if I enter security through the first class line, will I still have access to TSA Pre-Check?
If you enter security through the first class line, you will not have access to TSA Pre-Check. The TSA Pre lane is a separate lane and only there you can not remove belt, shoes, jacket, liquid bag, laptop. You can not exercise your privileges in another lane. Yes, as more and more people are enrolled into Pre sometimes using that lane is just not worth it. On the other hand, http://www.outsideonline.com/1957481/why-does-my-tsa-precheck-never-work claims
You'll still be able to leave on your shoes, but will have to bypass PreCheck's additional perks (like leaving your laptop in its bag).
I was unable to find anything on the TSA page confirming this.
Edit: http://www.tsa.gov/tsa-precheck/what-tsa-precheckR clearly says:
If you have a TSA Pre✓® indicator on your boarding pass, proceed to the TSA Pre✓® lane
It is a very interesting question whether you are allowed to use a normal lane if you have the Pre indicator -- I doubt anyone would care as long as you are scanned.
Not every airport has a first class security line and likewise not every airport has a TSA Pre. BUT you will still get your TSA Pre perks at these airports, but not first class perks.
Of course if you only fly from major airport to major airport then there maybe no benefit.
I have TSA Pre plus other priority options and usually just pick whichever line is shortest.