It's my understanding that you won't notice much difference in the quad cores unless you're running some games that can utilize it. Since the quad core phones will be new technology and my Rezound is working great, I'll be happy to let others do the testing and wait till they get the bugs worked out of those.
I'm not going to tell you that you won't have any problems with the Rezound because I think any smartphone can have problems and just because mine works great doesn't mean you won't have problems. However, coming from the Thunderbolt, for me the Rezound is pretty much trouble free. It is superior in every way to the Bolt and in my opinion, superior to about any other smartphone Verizon has right now. It works great for what I need, which most important to me is a good network connection when using the hotspot to tether to my laptop. When tethered to the Bolt my network connection was bad enough to be nearly nonexistent and in comparison, the Rezound is great. (Stable 4G with occasional drops to 3G in an extended coverage area).
If you require much heavy use and can't charge it frequently, you'll probably need the extended battery. Some people report no problem with the battery, but mine is a battery hog. I learned to live with that with the Bolt though, so I don't have high expectations for good battery life with any smartphone. I have no problems with the extended batteries and like the extra thickness for a better grip on the phone. It adds a little weight, but it's a matter of .85 ounces, not pounds.
One thing that was a nice surprise for me was that the Rezound can do screen shots, which I didn't know when I got it.