I just came from a Note 2 with V4A. But before using V4A on the Note 2, I had Cowon Z2 as a dedicated mp3 player which I bought solely because of Cowon's reputation for sound quality. The Cowon didn't disappoint, it sounded pretty good, especially compared to the Note 2 which I thought had very mediocre sound quality on its own. That changed when I installed V4A on the Note, which to me it transformed the sound into something better than the Cowon, so much so that I adopted it as my main music player. Well let me tell you, I think the M8 sounds pretty decent on its own (no effects whatsoever), but with V4A it's another animal altogether, it's like it unlocks the potential of the phone to what it can truly be. The M8 sounds a lot louder and cleaner than the Note 2 could ever achieve. For example, I used to drive up V4A's Playback Gain Control on the Note to 4X just to make it loud enough, which would introduce some noticeable background noise. The M8 does not need any additional gain, it's loud and clean with very little background noise. Additionally, I find that the sound tweaks I made on the M8 are a lot less than with the Note 2, and it sounds so much better. I ended up selling the Cowon, I don't need to carry it around when I get awesome sound from my M8. Oh, and the Note 2 is for sale right now.
So in a nutshell, I think it's worth it, especially if you install V4A on the HTC One.