I am in Turkey and writing this response from my 3G iPad using a Turkcel sim card. Each time I come out I purchase 1G for 39 lira ( this is ample for my use for c. 4 weeks) and I find that the speed is normally very acceptable.
Just make sure if you purchase a sim card that your iPad is registered here in Turkey, otherwise it will 'lock' after about 2 weeks and won't be able to be used until registered.