Granted I have had the Nitro 3g device for only 2 days right now but I must say I am pleasantly surprised by it. The speed varies according to the time of the day of course and slows down considerably during peak hours of 8pm - 10:30 pm . But this is something mentioned on their website as well, something to do with the number of users on their tower at one time, plus the speed decrease is still bearable. When you weigh it against mobility provided and always on connectivity the pros definitely outweigh the cons.
I had the Worldcall Thunder4 (4 MBps) connection before and I wasn't exactly disappointed by it. My only issue was that when there was a power outage, even after the generator kicked in and electricity was restored in the house, the internet and cable from worldcall were still out. It seemed that their service stopped when the power went out in the area.
As far as PTCL Nitro 3g goes I admit 2 days is not long enough to form a lasting impression but so far the going is pretty impressive. When downloading torrents I used to get a constant 390-420 kbps while using worldcall. Using PTCL I get between 220-720 kbps depending on the time of day. Yes the variation has increased but overall it takes pretty much the same time to download a file (large file that would require around 24 hours to download) using both isps (PTCL downloaded the file in 22.75 hours while it took worldcall 23.5 hours to download).
Here is the customary screenshot of the speedtest (I live in DHA Lahore).

Edit: There is one issue of slight concern though. The connection seems to randomly turn off all by itself sometimes. Not a big concern if I am on the computer at the time as the connection can be turned back on by just one click but might be an issue if I leave the connection on overnight. Any suggestions?