I brought the airtel 4g dongle+sim couple of months back. But its nothing like the ads might have seen on tv, speed is comparative to 3g not better
Cons:
1. Network not available everywhere only limited cities
2. You might have to purchase a new phone because only the new phones are coming with the LTE feature
3. Costly data charges
Pros: Will be good for future because the coming technology will be 4g but not currently now
Suggestion Go for the 3g packs, they are more value for money