I received the stroller packed tight in a box from my local running store, OK Runner. I was surprised at the size of the box, expecting something much larger. So was the guy working at the store who joked about how long it was going to take me to set it up.
When I got home, I opened the box and there were 4 parts inside: the stroller, and 3 wheels. It turns out that assembly was super simple, just unfold the stroller and pop the wheels on. The wheels are quick release, so they just snap into the axel and can be removed for transportation or storage. The stroller itself folds up remarkably well by just lifting a strap under where your baby sits.
Usage (Runner's perspective)
There is some adjustment I had to make running with a stroller. You must hold onto the stoller (obviously) so your arms don't swing naturally, which left me a little sore after a while. eventually I moved to pushing with one hand and swinging the other, which was a little better.
The stroller itself is very smooth. I was initially concerned with turning since the lead wheel is fixed, but it turns pretty easily (though sharp turns require minimal effort to lift the lead wheel up). Another nice feature which I initially thought silly: breaks. Running downhill means you have to pull back on the stoller to prevent your kid from careening down the hill ahead of you and crashing into the bushes or something. However, with the FIT, just apply some break and there's no need to alter your form to accommodate the stroller.
I am able to run as far with the stroller as without. There doesn't seem to be any (or at least very much) extra effort to push this thing, even with a 25 pound baby (not included). I generally use it for shorter runs, under 5 miles. The BabyJogger is extremely smooth, something I didn't feel with some of the cheaper alternatives.
Usage (Baby's perspective)
I'm not a baby, how wold I know? I'd ask the kiddo but he's more interested in leaves waving at strangers. Actually, I can tell he rather enjoys the run, even though he hates being in strollers. I think the extra fast stoller excites his inner speed demon. He doesn't like me pushing down the cover, because he's worried he can't see me and perhaps that I've left him rolling down a hill alone. But other than that, he's comfortable, and happy to ride as long as I'm willing to push him.
So the BabyJogger FIT is awesome. Smooth riding, no impedance to distance or pace, baby likes it. The only cons are a hefty price tag, but you get what you pay for.