You are absolutely right about the slow speed. Is it "usable"? The answer is yes and no, and depends on one's use case. Unlike other roaming solutions, PTCL Evo has by and large a great coverage area. Yes, EVDO Rev. B is not available everywhere, but then we have slower EVDO Rev. A, and then older CDMA. I have used PTCL Evo in far flung areas of Pakistan. In many places, the use was admittedly limited to checking emails, but in my use case it was indeed usable. In yesteryears, I have used Ufone and Telenor USB dongles in the field. In a majority of places, PTCL Evo gave a better speed. Once they roll-out reliable 3G network throughout the country, or we have a better 4G coverage area then the scenario will change.
Having said all that:
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If Zong is offering 4G service in your area, then it is the best in terms of speed. The price is okay (24 GB -- Rs. 1500 per month). A lot depends on the availability of 4G if one is always on the move as 4G is only available in major cities; even the coverage is patchy within cities.