MiFi and SIM cards

Is anyone familiar with MiFi / Personal HotSpots (preferably pay per use and no monthly plans) as well as pay as you go SIM cards for unlocked phones ?

I just had my iPhone 4S unlocked (by AT&T) and want to put in a new SIM card, but no more monthly commitments.

Also had an internet outage at home which makes you realize how much you rely on it, so I was thinking about getting a personal hotspot just in case this ever happens again.

Had a quick look around a few stores and also online, so there are definitely options out there, but does anyone actually use these and can tell me about the pros/cons and costs ?