Performance wise, the WP 8.1 is actually a really smooth OS that I've never ran into any problems with so far. The only bad side to the platform is not being able to download the same variety of apps you can on an android phone (hopefully can be ported soon).
The best part of the phone, in my opinion, is the speakers. They are not only loud, but also surprisingly clear and crisp. A lot of my friends always ask me if I'm using a Bluetooth speaker, when it's really just my phone playing music.
The screen is amazing. The viewing angles are great, and everything is just perfect. There is one issue with the auto brightness where it wouldn't lower to the lowest brightness in a pure dark condition and wouldn't raise all the way in the sun. This isn't just my device as one of my friends also experienced this. Hopefully this is a software issue that can be fixed.
Overall, the phone performs just was well as the regular Android or HTC Sense editions (same hardware). The browser could use a little bit of work because it's relatively slow, but that's why 3rd part apps exist.
If you guys have any questions about the phone, I'll be happy to try to answer them below. Thanks!