I owned an X from day of launch to literally yesterday, and in that time I can tell you my personal problems with the X.
1. Unlock Screen: Just like everybody else's complaints, mine was terrible, you would swipe the bar over, and it would stick and just not unlock. Or you would swipe it over and the phone would take a good 5 seconds just to bring up your home screen.
2. Motoblur or w/e you want to call it: Motoblur is so laggy on that phone its ridiculous, I immediately found myself installing Launcher Pro on the phone, followed by purchasing the full application.
3. Overheating: For whatever reason, the X when running multiple apps in the background followed by surfing the web, my phone would heat up in my hand, and by using a laser thermometer I found that the phone was actually upwards of 100 degrees Fahrenheit in my hand. As an experiment when I was having frequent overheating, I downloaded ATK just to kill non necessary apps/games in the background, I could actually prevent the phone from overheating.
Upon grabbing the Fascinate I noticed that the screen is far superior, its much easier to root and flash ROM's and such/ not that we have a full stable kernel yet but once we do it'll be great, Thats something you'll never get with the X. Touchwiz is much better than Blur, and on top of that bing isn't all that bad. In fact I don't much notice the difference, and overall the phone is much quicker. My Quadrant scores with the Fascinates custom ROM are even higher than the Nexus one with Froyo.