i have both. my personal phone is an incredible and work just replaced all our windows mobile phones with the X. i think alot of the comments in this thread are valid. the big screen of the X is very nice but once you pick up the incredible the screen on the X looks very pale and washed out, the AMOLED screen of the incredible is fantastic with vibrant colors. and sense beats the snot out of the motorola's launcher, i had to swap it out for launcher pro the first day. however for work (alot of emails etc..) the large screen of the X is really nice so it fits that role very well although motorola's exchange support is horrible, ended up having to use touchdown. one odd thing i noticed with the X is it will "freeze" or pause every now and again randomly just a second no matter which launcher i use (motoblur, LP, ADW). and it did this out of the box before it was connected to work's corporate systems so i know it isn't that nor is it an app causing it since nothing was installed (outside of the pre-loaded crapware). honestly had i not had an incredible i probably wouldn't of noticed but sense and the incredible is just so snappy and smooth i noticed right away. all that being said the X is a damn nice phone (the build quality and feel of the plastic is phenominal and it doesn't attract finger prints like the Inc) and has alot of posatives especially if motorola can fix some of the bugs which they claim to in their 2.2 upgrade.
i really don't think you could go wrong with either phone but if you have had both of them for more than a few days you do notice the differences between them really quickly.