I got the RMHD pretty much when it first came out. And earlier this week, I picked up the S4 (more on a whiim than anything else). Here's my list of pro's/con's
1) Battery life. Hands down RMHD wins. But then that's obvious. I can comfortably last 18-30 hours or so with RMHD. With S4, I get about 12-18 hours.
2) Look and feel. Personal choice really. I like the heft of RHMD. Some may prefer the very lightweight feel of the S4. Although S4 is made of plastic, it doesn't really have a "plasticy" feel to it. But the vote for me has to go to RMHD.
3) Display. no contest. S4 wins. But we're comparing an year old display to the latest/greatest.
4) Camera. no contest, S4 wins again. It's faster, more responsive and has more bells and whistles. Again, year old tech to the latest.
5) Music. No contest. S4 wins. I listened to hours w/o a single skip. With RMHD, it's skips galore. So nothing changed (headphones, songs, MP3s, Google Music etc.) and RMHD skips and S4 does not. So I'll have to blame Motorola for screwing something up. Someone mentioned that having at least 100MB free on RMHD may help. I think it helped me in a recent flight, but I still got skips.
6) Misc. I like that S4 gives me a lot more control over with a single swipe down of the screen. With RMHD, adding power control widget helped, but it took a few more clicks to get to what you needed. But again, some may find S4's designed to be too cluttered.
7) Moto's FAVORITES and SMART ACTION is a winner. I had to get PixelPhone to get similar functions. With RMHD, I never had to go looking...but now that I tried PixelPhone Pro, I think I prefer that over Moto's Favorites widget. I can find similar apps that do what Smart Actions did, but SA is pretty easy to use and configure.
8) I like that S4 gives me the current locations time and "home" time on the lock screen. As a road warrior, this function comes in handy. I also like that S4 gives me the option of two different wallpapers, home and lock screen. Yes, you can find apps that do this, but I like that it's built in with S4.
9) I recently did a FDR on my RMHD because Smart Actions started randomly kicking in, and I started to see a little bit of lag. Nothing terrible, but I figured it was time for a FDR. I haven't had the S4 long enough to find out what "gunk" this phone will collect over time. So this one is a TBD
10) Call quality. About the same. But I do like that S4 gives me the ability to change all the different volumes by clicking on the "star/setup icon" RMHD stopped doing this after the JB upgrade.
11) I haven't figured out how to remove some of the built-in icons for functions on the S4 (top right of the notification area). RMHD was much more clutter free.
12) Of course S4 is noticeably faster than RMHD. But again, comparing the ages of the phones, it's a given.
All in all, the dated (in cell phone tech years, anyway) RMHD stood up quite well to the S4. Hopefully, this helped give it some perspective.
PS: I did order the 4200mA battery from Seido Online. Hopefully, that will give it some heft and the battery life I got used to with RMHD.