Someone known to have good leads inside HTC has spoken on Twitter about the coming 4.3 update for the HTC One, and he believes we'll see it in a few weeks. This matches up with information we've already heard, but there's some more information here that we think makes it newsworthy.
According to @LlabTooFer, we will see the US versions of the HTC One get the Android 4.3 update with Sense 5 soon. After this happens, International versions should prepare for the Android 4.3 with Sense 5.5 update. After the release of the HTC T6 (the HTC One Max), all HTC Ones, the Butterfly S and the One Mini will see Sense 5.5 with the 4.3 update. Specifically mentioned as not getting the 4.3 update were the HTC One S (all variants) and the HTC EVO LTE on Sprint.
There's not much information about Sense 5.5, just this rumor that we should see it eventually. We're looking forward to finding out more, and you'll be the first ones to know.