The only drawback that comes to mind is that some messages aren't delivered. Bank texts, and those security codes that Gmail, Facebook, etc will send to reset your password.
Although this doesn't happen to everyone.
[MENTION=63376]shaheerk[/MENTION] can give his experience as he has ported to different networks.
If you're on prepaid, porting usually just takes 24 hours.
For postpaid it might take a little longer.
Postpaid can take anywhere between 3 days and 10ish... You need to make sure your bill is paid (and some excess is also paid) for postpaid porting to go through. And one fine day, it'll happen.
If you're on postpaid, convert to prepaid, and it'll happen in 24 hours or so. Much faster than waiting a week.
As for the drawbacks: one is you don't always get messages from the short codes/SMS services within Pakistan - as a workaround, these companies usually say "send an SMS to xyz number and you should begin to get messages".
The second issue is that if, for whatever reason, you don't like your current network, then you can't port back before 60 days.
I just moved onto Telenor. I get calls dropped to 2G with Telenor as well (I thought I wouldn't), and it's no different - except one operator may not fall back in an area, and another would. However, they allow dual transfer mode on 3G. While I get signals at home for making calls, signals at home also vanish randomly and I have to toggle airplane mode as well.
In short - you most likely won't be satisfied with any operator.
Oh, and as for Telenor - where they have carrier aggregation, speeds are plenty fast - except when carrier aggregation doesn't kick in, or the signals are just awful (quality, not quantity). Not much you can do about that (as I am). I plan to move back to Mobilink despite the issues (because Telenor, too, has issues).
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