My experience T-mobile
I had fairly poor signal in my house normally 1 bar -108dbm which stayed surprisingly stable, pretty much always the same.
Speeds roughly 5mbps down/ 3 -5 up
I checked it quite often, because I'm anal like that.
After 4.4.2 update I never get better than -113dbm sometimes -115dBm, phone drops to reading 0 dbm but LTE data connection is still working.
Occasional drops to HSPA+ , with very similar speeds to LTE
Speeds lowered 2-4 mbps down / 1-3 up
At office where I generally have a great signal --88dbm I'm now at -95dbm
But speeds are identical, if not a little faster
23mbps / 9 mbps. with beat my old record of 22 mbps / 8mbps
The good news is I'm not left with an unusable phone, the data in my case hasn't gone out at any point.
HSPA and LTE has always been about the same speed in my apartment so if it drops to HSPA+ with more bars I don't mind.
Also this is completely subjective, but I used my phone for about 2 hours of calls last night and the voice quality in my otherwise low signal apartment seemed to be better, less audio issues. Don't know if that's a result of the new modem or not.
It seems to me that this only really affects people who already had a poor signal in certain places, particularly T-mobile users.
While I am more than comfortable rooting and flashing the old modem, I used to be a flash addict.
I was too enjoying using my nexus 5 stock and I'm disappointed that this update which improves so many areas of the phones, slightly degrades network performance for me. As it doesn't cause outright dead spots in my usual usage areas. I am going to stick with it for awhile as see how it affects my usual real world usage before doing anything. Because it's nice to just use a phone and not worry about flashing stuff constantly.
Luckily I unlocked my bootloader first thing when I got the phone, because I know that eventually one day I'll get bored and start tinkering with custom roms lol. So if I decide to root, install a custom recovery and flash the old modem I don't have to reset my device.
For those who went back to the old modem on an otherwise stock device, have you noticed any issues using the old modem?
Sometimes using an older modem can mess with voice calls for instance, which are actually still important to me.
I don't want to lose what seems to be superior voice quality from the new modem, but I'll need to do more testing to confirm that suspicion.