The Z1 (for it's time) had really good battery life, not so great in comparison to the Z2 and Z3 which had exceptional battery life. The Z3+ I'm hearing is about average on battery because they dropped the size a bit. As for the UI and software, it's very very clean and elegant. Much more attractive than Touchwiz or LG's UI. Sony doesn't heavily skin their UI, they just do enough to make it unique to them and are classy about it.
As for performance, one thing I've noticed about Sony's engineers, they are pretty damn precise and not sloppy at all. I've repeatedly watched them release android version updates that ran just as well as my Nexus devices. Most recently when they released 5.02 they took it upon themselves to internally address some of the glitches that were complained about from 5.0 to 5.02 that Google themselves took until 5.11 on the Nexus.
I'm glad they're finally taking steps (if this leak proves true!) to address the fact that the Camera sensor on currently existing Xperias is actually worse than the sensor they themselves license to their competitors lol. Music is also a treat on Xperia. Best damn music app of any mainstream android phone. Speakers aren't quite as loud as HTC's but close enough. Not really a sticking point to me, I always use the headphone jack either on the go or in my truck. Their photo app is quite nice as well but I don't bother with it because Google Photos syncs better with my tablet, and I have no desire to use Sony's own cloud service.
Anything else you want to know just ask, I've been using Xperia phones since 2013. I still happily use my Z1 as my daily but I've had the Z2 and Z3 as loaners to play around with for a few weeks each because I have a family member who's a sales rep for Sony's mobile division
The only reason I haven't upgraded is because I've been waiting for them to significantly upgrade the camera. I'll admit I was tempted by the S6 but all of the complaints going around turned me off. I'd like to finally switch phones this Fall so if this leak pans out my top 3 picks are going to be the Z5, Nexus 2015, or possibly switching over to the next iPhone.