I am sure, like others HTC One M8 users are also enjoying the Android 5.0 lollipop with sense 6.0. For those users who are waiting for the latest Android 5.1 update, will have to wait for few more months to enjoy the new software update with sense 7.0.
According to tweets from HTC UK, the flagship One M8 will be treated with latest Android 5.1 in 3 to 4 months which is August 2015. The latest update will not be simple update for HTC One M8 users because it will be bringing Sense 7 UI along the latest update.
One release of Android 5.0 lollipop, HTC put a bar before itself to release the update with sense 6 UI within 90 days but after number of delays and unmet deadlines HTC finally released Sense 6.0 with Android lollipop a few months ago for the last year flagship One M8 and M7 and also released update for the Google play version of M7 and M8.
We have to wait to see whether this time HTC will be able to follow the deadline or will again face the unmet deadline issues like Android 5.0 lollipop.