I pre-ordered one today as well, but I did it off-contract as I didn't want to extend my commitment another 2 years. I currently use a GSM unlocked Galaxy Nexus on T-Mobile and have a EVO 3D on Sprint. I loved the original EVO but have never liked the EVO 3D and then when the iPhone killed Sprint's data network I tried T-Mobile with the Amaze and really liked it. Currently the Galaxy Nexus is my daily driver, but I figure if I'm going to keep my Sprint line for another year I might as well have a phone I like on the network.
I really wanted the HTC One X but have no interest in moving to AT&T (my wife has her iPhone on AT&T and it's terrible). The One S on T-Mobile has too many limitations (no NFC so no Google Wallet, which I use a lot, small amount of on-board storage and not expandable, lower resolution screen, non-replaceable battery) that don't make it a good replacement option for my Galaxy Nexus.
The EVO 4G LTE has the features from the One X I want (the screen, the improved camera) and some additions like the kick stand, the microSD slot and the physical camera shutter button that for me actually make it better. I'd be a lot more excited about my purchase if I thought I'd see some improved network performance (Sprint 3G here in Chicago has really become horrible over the last 12 months). Who knows, maybe I'll be surprised. I've heard they've been testing 4G LTE down in Kankakee, IL which isn't far from Chicago, so maybe we'll actually have it here in the city before the end of the year.
If the EVO 4G LTE sucks, then no worries, I can unload it on eBay and at that point I'd probably just pay the ETF for my remaining year on Sprint, I can't imagine there will be a better phone in the next year that would make it worthwhile staying on (no, the Samsung Galaxy III isn't an option for me). But I'm hoping the phone itself is good enough to convince me to live with Sprint's slow 3G coverage until LTE arrives.