My Thunderbolt is going on 3 years old now, but I've yet to find anything else I really want more. But it seems like it's getting slower and more of a pita. I have a Razr M for work and it's a nice phone and fast, but I don't like the interface as well as on the HTC.
Is this finally the One? I was thinking it might be between this and the Moto X.
I would definitely say to go for the One. Your only potential regret is that you'll see a Samsung S5 and other various new phones announced in a few months, and since the One is already almost half a year old, you may feel a little left behind.
Between the One and the X, I tilt strongly toward the One. The X is an absolutely fabulous phone, and my wife is getting excellent battery life with it -- however, it just feels more like a "phone for the masses." It's an excellent "everyday" phone. The One on the other hand -- for at least two reasons -- just feels more tailor-made, first because of the design touches (though watch out, the soft aluminum body scratches easily) and second because of the Sense UI. It's my first HTC phone and I'm very happy with it. I think I've rebooted it just a few times in weeks, and it has remained very, very fast and responsive.
My favorite feature on the One is probably the screen, which is just a joy to use. Even though technically (I think) both the One and X have 4.7" screens, the One feels larger. I've got smallish hands, but have no problems using it (YMMV). I also heard some concerns that a 1080p screen is a battery drain but that's not been my experience -- most days I get home with more than 80% battery left, but then again I switch to wifi at work and generally keep an eye on power usage over time.
An important consideration is whether you'll pay an upgrade/discounted price for your next phone or full price. I lucked out and got the One while it was $50 (upgraded) -- I don't think I'd pay $550 or $600 though. At that price I'd probably wait until early next year, when the One is either under $400 or maybe save up for the next latest and greatest!
-Matt