I haven't heard of any problems yet. But yes, if you take up a limited plan, the download limit is sure to get exhausted before even you blink your eye. The upgrade that MTNL has done, is a business strategy that will ultimately earn them more money. With such high speeds, most people will exhaust their download limit and then have to pay more for the extra MBs they download. So, it is not advised to take up a 2 Mbps plan right now.
But a rumour has crept out that MTNL will eventually upgrade all plans to 2 MBps (including the unlimited ones). Either you could wait for the upgrade, or I would suggest taking up the 590 NU plan. It's the best that MTNL offers.