It frequently freezes, especially when switching apps, and lunching apps is often way slower than it should be. And I doubt it's the phone, because the same thing happens on my 1520 also. In fact, part of the reason why I bought this phone was because I thought the 1520 was getting too old. The Idol 4S actually loads many websites significantly slower than my 950XL, which kind of shocks me. The awful performance seems to be a W10M thing overall, because this phone operates barely faster than the 1520. It was quite a disappointment.
And RumoredNow is correct. Flashing the T-Mobile ROM will not lock your phone. However, you will lose just about all other functionality besides basic call, text, and data. Call forwarding, gone. Visual voicemail, gone. Using third-party voicemail services, gone. Mobile hotspot, gone.
Although on the plus side, my camera no longer forces random reboots like it did on the Lumias, so the reboot issue must be related to the proprietary OIS that the Lumias use. I guess that's something positive, right?