Just yesterday, Google started to roll out the latest dessert from the Android team, which is Marshmallow. Although that's not really a dessert now is it? Anyways, Android 6.0 Marshmallow began rolling out yesterday to the Nexus 5, Nexus 6, Nexus 7 (2013), Nexus 9 and Nexus Player. T-Mobile also noted that they would be rolling out the update to the Nexus 6 this week. However, it appears that the update has taken away signal for some T-Mobile users, at least according to a post in r/TMobile. Many are reporting poor reception compared to what they had on Lollipop, and not getting LTE in places where they had previously with Lollipop.
This is not just for T-Mobile customers though, a T-Mobile employee chimed in on the thread and stated that they are also having an issue with signal after getting Marshmallow on their device (they didn't specify if it was the Nexus 6 or Nexus 5). The employee did also note that he would be mentioning this internally, so hopefully T-Mobile and Google can get together and see what's going on here and get a fix out as soon as possible.
Also according to the thread on Reddit, this isn't affecting everyone on T-Mobile. As there are a few people that flashed on the Nexus 5, and haven't had any issues just yet. Which makes it a bit strange. We'll be sure to keep our eyes and ears open to see what we can come up with. Hopefully there is a fix in-bound, as it would definitely suck to flash Marshmallow and then be stuck on 3G in more places, where you used to get LTE, especially in this day and age as 3G is super slow.
The factory images for Marshmallow went up on Google's website yesterday for the Nexus 5, Nexus 6, Nexus 7 (2013), Nexus 9 and Nexus Player. So if you really want to get some Marshmallow onto your device, you can go ahead and flash the factory image now without any issues, hopefully. If you're on T-Mobile, especially with the Nexus 6, it might be a good idea to wait until T-Mobile rolls out the Android 6.0 update later this week. As the radios could indeed be slightly different.