I ordered a Straight Talk AT&T MicroSIM and $45 unlimited card online and tried it out with my roommate's old Galaxy Note. Keep in mind that if you use the promo code "SIMSAVE" you get $10 off and free shipping, so you basically get the MicroSIM for free.
I live in Indianapolis, IN, but I tried out ST around Toledo and Sandusky, OH as well as in the suburbs around Detroit, MI while traveling. Excellent signal the whole time (same when I got back to Indy), including inside a huge convention center in Sandusky. Signal was often stronger than my VZW GNex and my mom's Sprint GS3 (not saying much, I know!). Speeds tended to be around 2-7 Mb/s down, 1-3 Mb/s up. Not quite LTE speeds, but I didn't even notice when doing typical web browsing. Like someone else said, you'll probably only notice when downloading large files/apps or when you wait longer for the buffer whle streaming video. Overall I was very impressed by the service.
Only downside is that I activated the service ~2.5 weeks ago and I expected to have my N4 by now so I could cancel VZW and port my number. Now I'm thinking my 1st month of ST service will just about be done with before I get my N4! Kind of a waste of the $45 I spent on the service, but it's better than receiving the N4 and having no idea what my coverage would be like before I ditch VZW! Only thing I'm not sure about is how well the signal will hold up while I'm in the hospital all day at work, but my GNex doesn't do too great with that either so it can't get much worse.