I had the EVO for a year and just switched to the SGSII a month ago. I'm absolutely love this phone! As someone pointed out in another post, the extra screen size really is noticeable.
My EVO was and is awesome. Before I upgraded to the Samsung I was running CyanogenMod 7.1 on my EVO, and it was working really well. If you haven't tried this yet with your EVO, give it a shot before you upgrade - it might just be the ticket for you.
Things I miss about my EVO? The kicstand and FM radio. I used the kickstand daily for video stuff and for a bedside stand, and the FM radio daily as well. I now use TuneIn Radio Pro for local radio stuff and it works just as well, just pulls more on the battery since it's using data instead of native FM.
Speaking of battery... my EVO wasn't bad, especially with the correct settings, but the Samsung just rocks! It's definitely better than the EVO. I'm using these battery packs
2000mAh Battery SPRINT EPIC TOUCH 4G GALAXY S II I9100 SAMSUNG Vibrant S t959 Verizon Fascinate AT&T Captivate i897 I had great luck their 1900mah packs for my evo, and have had good luck with the 2000mah packs with the Samsung so far.
Best thing about the Samsung is it's just freakin' blazing fast! And don't let the screen resolution scare you off (when compared to some of the newer ones coming out) - it looks crisp and clear and vibrant! Sound out of the speakers is better than the EVO. The Camera is WAY better. I had gotten used to using Camera360 on the EVO, but I seriously think the stock camera on the Samsung is better.
Sense UI vs. Touchwiz is a toss up. I'm currently using ADW Launcher EX and it works well.
Long story made short... I started out with the standard black one and exchanged it for the white version. The white one is much less slippery in the hand than the black one. It also shows fingerprints and smudges much less than the black version.
For what it's worth... I love(d) my EVO, but I'd never go back.