I have the following: 635, 640 and two 920s. The 640 was my daily driver while I tested with the 635 and one 920 (the other being a control to test speed). I was happy with 586 enough to load it in my 640.
I have run into some issues though. Edge loves to crash. This seems to be an edge issue as my desktop version will crash in the same manor in the same sites. I really do miss the download option when streaming video.
Second. If you plan in loading apps on your SD card, get an U3 rated SDXC. Anything less and you will be waiting for things to load.
Third. WordFlow has some bugs. Do not adjust your text size settings from default. This somehow broke the Swype like mapping of WordFlow making it unusable. Setting it back to default seems to have fixed the problem, but it is not as easy to type as in 8.1. Oh and for whatever reason, WordFlow does not work in the app store and some other very specific categories of text entry such as passwords.
4. Battery seems to be very dependent on your inbox. I have half a dozen accounts for work. Google and exchange mail accounts really drain the battery. Yahoo and Hotmail are not bad. If you need more battery life, change settings here.
5. Hardware support is awesome. Bluetooth keyboard and an arc mouse.... Who needs continuum?
6. Want to see something really neat? Take two phones, load one with 8.1, the other with 10. Turn off cellular but keep WiFi enabled. 8.1 device may last two days. 10 device can last up to 12 DAYS! It looks like W10 has an incredible low power state of cell is turned off.
7. Using various benchmark apps, pre 581 builds had a 10-15% hit in stats compared to 8.1. This was right across the board, but stood out especially in CPU, RAM and graphics scores. Post 581 builds have negligible difference.
8. Screw you MS for removing Lumia Refocus and Panorama from the app store. Both will stay in your system and work if upgrading from 8.1, but if you uninstall them, you can not find them in the app store. If you fish through your app library, they will the store will say you need Amber firmware or better. But I'm in Denim. Not sure what is up with that but I really need the all in focus feature in Refocus for macro shots.