[HTC Rezound] Neo:
Let me be the first to welcome you to XDA! Anything available for smart phones will most likely be found on this site. This community is very helpful in answering any question you may have although the site does have specific guidelines to follow before asking questions. Seeing as how this is my help thread, I do not mind answering any questions presented here regardless of whether it has been asked previously somewhere buried in the forums. So feel free to bring your troubles here any time.

Seeing as how the Rezound is a global device, it can in fact be used on other carriers. I honestly can not tell you however on whether the data speeds will be on par of that of LTE nor to process for which you would switch to that new carrier as I've never done so nor spoken to anyone who has converted to another carrier. I hope that someone who has had such experience will see your question and answer it for you.

Regarding any possible Mms troubles, as far as I know, other carriers don't have a problem.

As far as rooting is concerned, I can most definitely help you with that process. You won't believe how much faster, smoother, and battery efficient your Rezound can actually run with some work on your end. The following is how I'd lay out both speed and performance from slowest to fastest:

Stock HTC Sense - Locked, bloated, and sluggish. Gets so bogged down that trying to make a phone call takes 30 seconds for the phone app to launch. Keyboard is incredibly slow and just stops working every now and then.

Rooted Stock HTC Sense with bloatware removed - Freedom to breath a little! Phone feels faster, apps load quicker, keyboard still feels sluggish and the phone just feels like it could be better. Over time with such freedom you add more apps from the playstore and now you're back to feeling slow again.

Custom Sense Rom (ICS) - You made the virgin voyage/gut wrenching leap to a custom rom and it actually worked! You have options available to you that you didn't have before including wireless tethering which was originally blocked to you. The phone feels much better, faster, has mods in place to make it feel awesome. Still, over time, you can't help but feel like it could be faster..

Stock AOSP ICS - HOLY SMOKES THIS IS FAST! Where have you been all my life AOSP? Welcome to a sense free environment where your roms are no longer over 500mb and are around 100-200. You are free of an over-sized system with a battery hogging sense GUI. Camera and camcorder work flawlessly in this version also. Visual voicemail and Verizon specific apps aren't working anymore... But you can easily replace it with Google voice. You dance around trying several different ICS AOSP version roms and they are all cool, but eventually you get addicted to making the phone faster and smoother... Is it possible??

Custom ICS Roms: (Android 4.0) Yes it is! So much faster but how? The developer has added init.d mods and tweaks to increase the performance of the rom. It can't get better than this..

Stock AOSP JellyBean (Android 4.1-4.2)- I was wrong.. This is smoother and even faster! Everything works but camcorder every now and then goofs up.. That's odd.. Rarely happens but suck swhen it does. Camera works great.

Custom AOSP Jellybean (Android 4.1-4.2) A little better than before. the developer added mods to once again increase performance.

Stock AOSP Jellybean (Android 4.3) Now this is the life... I didn't think it could get even faster but it did. However, Camcorder only works half the time for me?? Camera works great. The developers are all over this platform right now and working to make it better.

Custom AOSP Jellybean (Android 4.3) F.A.S.T. I just flashed TACHYON 1.1.5 (Self Neo plug ;) ) and I'm in heaven. This is ridiculous how fast my phone is running now. It's like I upgraded to a new device!! Camcorder still sucks but who cares?! /neo backpatting

Stock AOSP Jellybean (Android 4.3 ION) Slightly faster which I thought was impossible... Camera and Camcorder stopped working.. I guess they could no longer keep up with the speed? How odd.. Oh well!

Custom AOSP Jellybean (Android 4.3 ION) TILT. I have no words to describe such perfection. Please give me some time to comprehend..

So there you have it :p Your path to awesomeness. There is a lot to be done with rooting which I will cover in just a little bit. Search the general threads and the Q&A sections to educate yourself a bit on what needs to be done to get you to the peak of cutting edge development.