I use it and don't mind it, I get around 2 days of battery with moderate use and some calls. Wi-fi is always on, localization services are always on, mobile data (3G) is always off. It lasts me more than one day even if I keep using the phone so I'm fine with it.
I would say it 'eats' 10 to 50% more battery per charge depending on usage. If you have it mostly in standby you are more likely to notice the battery hit. If you keep using the phone for one thing or another, it won't make much difference.
With 4.2.1 it seems to hold pretty well while being very snappy. The official 4.1.2 follows the same trend (lasting around 2 days with the same use), but I really liked how fast it got with Air Kernel. I usually have around 200 MB free.
I'm currently using Rasbeanjelly (
http://forum.xda-developers.com/showthread.php?t=2028718) with Air Kernel #270 (CM,Voodoo Colors
http://forum.xda-developers.com/showthread.php?t=1469842) and noop+SmartAssV2 setup. So far battery seems to hold up fine through the day. The only real downside I got from this was the occasional sleep of death (while using the phone, very weird, I would report it to the maintainer but I can't as I'm new around here), a FC when accessing Chrome settings (seems to be related with google sync feature) and the less stellar performance when changing orientation (same kernel and official 4.1.2 ROM doesn't have the issue).
Although it is handy sometimes, I think the worst aspect is the 'nagging' that Google Now has. Last time I got a "WTF" expression was with it saying that I walked 40Km that month more 20 than the last month, a nice feature, but completely unexpected, I would like to have a proper access to that kind of cards and the information that they provide... If you search weather it won't give you the same weather card shown automatically by it (at least for me).... one of these says it will be ready to take over the world...