It's really a trivial cost compared to the DynamoDB price before. It used to cost about $3600/month to get 5000 writes/sec. Now in a bit more than 2 months you'll have broken even on the reserved price compared to the old DynamoDB price, and in a bit more than 3 months compared to the new DynamoDB price, for the 1 year investment. Any company using Dynamo for anything serious will probably rush to get the reserved capacity, since they've probably been spending a multiple of the reserved cost per month anyway.

But 70% for 1 year is an amazing price break when you think about it.

Run your project for a month, analyse the usage, and reserve that same capacity for a whole year, for the price of only 3 more months usage!

If your project has fairly consistent (or growing) usage, and is going to run for more than 3 months, it really doesn't need much more thought.