First, I will never be Eris-less. I plan to continue using the Eris as a phone when I need something smaller and it will probably remain a dedicated music and book player for me. It is and always will be a great little device. My girl also still has her Eris so until she changes I can't be Eris-less.
As for the Fascinate it is really nice, HUGE, but really nice. With its specs it's obviously faster and it has a lot more memory. I reloaded all the apps I had on my Eris and still have over 60% of my memory left. The screen is nothing short of stunning and I am having trouble charging it because I can't stop playing (little kid with a new toy and all that) with it.
For anyone that loves their Eris, is no longer attached to Sense, wouldn't mind going bigger and has an upgrade the Fascinate is a really good choice. IMHO, of course.
Did you get the Fascinate at the end of your contract or what???