I have been using an MTNL 3G connection on my iPad since May last year and I am quite happy with it. Currently, I am on the Rs.4500 pre-paid plan which offers me unlimited data for 6 months. It's due for renewal within the next month and I see to my disappointment that MTNL has withdrawn it. These are the current pre-paid plans offered by MTNL Mumbai
The bad news is that there is nothing as good as the unlimited 6 month plan. However, there are still two decent plans for long term users - 3G 1190 with 9 GB for 90 days and 3G 2500 with unlimited usage for 60 days. It compares favorably with Airtel which has no unlimited plans at all, and does not seem to have too much choice for pre-paid users other than 'charge and use'
I have mentioned that MTNL works for me because I actually travel/roam extensively and I find my iPad working well on BSNL networks in small towns, on trains and even on the road. And they now offer online recharge like all the private operators.
I definitely find myself consuming more data on the iPad than my phone, so the unlimited MTNL plan is worth a shot if you are going to be travelling extensively in the next one or two months.