Well, I haven't played ESPN yet, but I've played every other 2k game extensively and from what everyone seems to be saying, it beats the h*ll out of 2k3. I liked 2k3 a lot.
If you read my Fever review post, you know I think Fever is DAMN fun! Read and lead is the new standard IMO. It won't catch on right away, because everyone will be out there adjusting umpteen sliders to tweak ESPN and Madden players will invent new and improved popcorn or whatever in owners mode. Fever doesn't have sliders and it doesn't have owner's mode. (If I want that much control, I'll buy a text sim). What it does have is an awesome and complex new passing scheme that makes you concentrate on reading defense and putting the ball in the right spot with the right strength at the right time. Simple as that. It, in a word, is innovative. AND... it works beautifully!
So, Fever is a FUN game! Something tells me if another game came out with Read and Lead, it would be like the second coming, if you get my drift.
I think NFL Fever 2004 is a very fun game. Also liked 2k3 and I'm sure I'll like 2k4 when I get it. But, I have a feeling I know pretty much what I'll be getting with ESPN. I was SURPRISED by Fever!