[T-Mobile Samsung Galaxy S6] whittikins:
When a service or app is bugged (like "Cell Standby") and either [1] truly draining the battery or [2] appearing to drain the battery in the Battery stats (due to a display bug), it's showing as the top drainer as we have all seen. When you FIX (or address, rather) that issue, something else is going to fill that top spot. It doesn't mean the new top item is draining more battery than it was previously. The stats have just been adjusted/corrected. Maybe "Play Services" was 8% before, but now it's 20% after the fix. That doesn't mean it's draining more now than it was previously.

I think in some cases, the "Cell Standby" is just a battery stats bug because some people have 30%+ "cell standby", yet they still get 4-5h of screen time over 15h and the phone sleeps properly. However, if "cell standby" is not showing as #1 anymore AND your phone isn't sleeping, then there's obviously still an issue (which many of us have witnessed).

Honestly though, I think that some people make it worse (and more difficult to diagnose the true problem) by messing with wakelocks and blocking services that they don't fully understand. A lot of services interact with each other behind the scenes, so blocking one may cause another one to keep the phone awake while it looks and waits for the one you blocked. I'm not denying there's an issue with the S6. There definitely is. However, the best way to troubleshoot it is to leave the core services alone and monitor the wakelocks (the #, frequency, and "time held awake") for a couple of cycles. Compare it with other devices on Lollipop which aren't experiencing any battery issues. Don't go messing with system services and wakelocks just yet. Use it normally but monitor things before making drastic changes. Otherwise you will never find the real problem. If there's nothing obvious you can find, change things like Wifi, bluetooth, cellular data, wifi calling, volte, and even remove the SIM for a day if that's possible for you. If it happens without a SIM, put it back in and try something else.

I personally haven't rooted yet because I'm looking forward to Samsung Pay. Therefore I can't really track the wakelocks like I want to. I might give in and do it though if we don't hear any news about Samsung Pay in the next few weeks.

However, to add my 2 cents about what I've noticed, the Cell Standby went from 30%ish to 2% after I disabled Wifi Calling and VoLTE (and rebooting, recharging). Today I have 44% left with 2.5h SOT (texting, email, reddit, web browsing, etc using Auto-brightness in a fairly bright office which is about 50% brightness -- but No gaming or videos) and uptime of 10.5 hours. At this rate I should reach close to 4.5 hours SOT and 18 hours uptime. I haven't changed anything else except disabled the Edge features (leaving People Edge ON). Supposedly using the "information stream" keeps the digitizer active at all times, which drains your battery a bit. I don't like the feature anyway, so I sure don't mind having it off.

I'm on 5.0.2 and took the OTA the first day I got the phone. I have NOT done a factory reset at all. I have all syncing turned on (2 email accounts, 2 calendars, all Google services/apps) and all Location/googleNow turned on. I also use Life360 which is always on and sharing my location at default settings (at least once / 15 minutes). I've listened to about 30 minutes of Google Play Music (streamed over BT in car). I have Facebook and FB Messenger installed as well. Here are my Battery drainers:

Screen - 16%

Life360 - 7%

Android OS - 5%

Google Services - 4%

Android System - 3%

MailWise - 3%

Hangouts - 3%

Cell Standby - 3%

Facebook - 2%