Me too got the T-Mobile M9 today, got the S6 10 days back and had canceled my M9 unlocked order since I had changed my mind. But couldn't control myself and bought the M9 from T-Mobile store today. Got to say I'm impressed.
1. Build quality, looks, feel of holding in the hand - Got to say M9 still feels so much nicer, premium to hold in the hand, more comfortable as well. Even though S6 stepped up a lot still feel M9 is the best.
2. Performance - Both are awesome and works well. Though had M9 only for a day personally will take Sense over touchwiz any day. Trust Sense and M9 to perform better over a period of time than S6.
3. Display - Both are great, some say S6 is the best and you can't use any other phone after using it ( Phil mentioned in today's podcast as well). I agree that S6 is the best but I was equally impressed about M9. At least I don't have any issues with M9 display and perfectly fine for a flagship.
4. Battery life - Well don't know about M9 since just got it today but honestly I don't care. Have fast charger in my car, home etc so doesn't bother me on both M9 and S6.
5. Sound - Well no contest, M9 by a landslide. But got to agree with others saying it's not that impressive compared to M8.
6. Camera - Well no contest, S6 by a landslide although haven't got the update on T-Mobile and haven't tweaked the camera setting still. But either way I'm not expecting miracles and probably not going to beat S6 or G4. That's fine for me since I'll be swapping between the S6 or iPhone 6 frequently.
7. Overheating - Got little warm while downloading lots of apps from play store and at the same time doing other tasks. Even S6 gets hot certain time so the overheating issue is a non issue for me. I'm not running benchmarks or I'm not a huge gamer so I'll be perfectly fine for daily usage.
8. Themes - Definitely better theming support and lot better themes with M9 than the S6. But that will improve with S6 as time passes so no big deal.
Conclusion:
If camera is important and you just don't want to worry about your photos after taking every shot then buy S6 and move on.
Otherwise, the M9 is a great phone and you will not be disappointed. You have to hold this to know how premium it feels. I wrote it off as well after reading all negative reports but had forgot how good HTC devices feel and how well sense performs. Give it a chance, go to your store and hold it, try it.
just couldn't resist could you @Habiib 
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