Data point:
I recently used O2 for several days in czech republic. I purchased the sim card on ebay in advance, as I was arriving in a smaller town (cesky krumlov) and wasn't sure I'd be able to easily find a card when I got there. Worked well, refilled from both a top-up card purchased in a convenience store, and also via an ATM machine. No issues using it in my huawei e586 mifi, coverage and speed were pretty good. All the text messages confirming the top-ups and plan selection where in czech, so it's helpful to have some other hotel/public wifi available to translate those when setting it up.