I was waiting on my upgrade until the X came out and then was going to decide between the X and the S4. However My Razr became completely nonfunctional after being dropped one too many times, so ultimately the need for a functioning phone now is what stopped me.
I picked up the S4 about a week ago and don't regret it at all. I do like the size and form factor of the X, and while removable battery is nice I could have lived with out it. Never thought I really needed micro-sd slot that badly b/c until I got the S4 I never put music on my phone. I am somewhat of an audiophile, so don't choose to listen to mp3's through an average phone DAC. However when I found out the S4 plays Flac files and is plug and play with my Fiio E!17 amp/DAC without root I was very happily surprised. Now I have a 64gb SD card almost filled with FLAC files. Maybe the X will have these capabilities, but with limited memory, wouldn't do me much good. I thought the S4 might be a little big for my liking, but I am enjoying the screen size and overall it is a great phone. Having a spare battery eventually will be a plus. The X looks like a great phone, but I don't think i will regret my choice at all when it comes out. The build, size and features look nice, although after the initial novelty wore off, I don't know how often I would really talk to my phone instead of just picking it up.