What is the cheapest and best mobile network in the Czech Republic?

I have recently moved to Prague from Ireland and I will be switching to a Czech network. Has anyone got any views on the phone networks over here, which is the cheapest or the best value, which is the most reliable, that sort of thing. I will be on pay-as-you-go for the first while at least, so if you have any good/bad experiences please let me know! Thanks!