[Effendi]

With CallBlue you don't have free incoming calls in CZ ($ 0.30/min), so it could be a good option for outgoing calls and SMS (cheaper), but not much for receiving them from the US. They are free in Germany and Poland, not in Austria and Slovakia ($ 0.30/min).
Probably Travelsim could be a good idea, it's cheaper to buy and you have free incoming calls in CZ and the countries around (except Hungary, Slovania and Italy). With United Mobile you have more free incoming countries, but you pay more for short calls.

For my trips to CZ I always used my Oskar card, now Vodafone, I think it has mostly 1800MHz coverage in Praha (Meir correct me if I write bullshit!) and I never had problems with it. Unfortunately mine has expired and I don't like Vodafone very much so last month I bought an Eurotel Go just to try another service, which was ok, also on 3G. I think they are all good, maybe Vodafone is still a little cheaper but still has SMS interconnection problems with foreign numbers (IoM cards don't send/receive SMS with Vodafone/Oskar, no problems with Eurotel).