I just installed one about 2 weeks ago. Noticeable improvement in mid-range performance, but not so astounding it would blow your mind. I don't have any issues with getting up to speed when I punch it. I like it. Took about 2 hours to install, and I wasn't in any hurry at all.
I've also gone to running premium gas, and there is a station a few miles away that has it ethanol-free to boot. My daily commute is about 100 miles roundtrip, about 30% on the expressway, the rest is county miles (~ 6-70mph) and 10 miles of stoplights. I'm getting just shy of 17mpg.