Let me rephrase that. Is it worth getting any sort of plan solely for Pokemon Go?
I'd need a new phone too since mine isn't compatible.
For Pokemon? No. But Project Fi is flexible in that you basically only pay for what data you use, and they credit money back to your account for data you don't use. I only pay $30/mo for 1GB of data and most of the time they credit me back for 75% or so I don't use. And if you go over they don't charge you any more than you're charged for the first GB. So honestly no reason to get anything other than a 1GB plan. It also rides off the Sprint and T-mobile networks so where one has poor service the other can make up for it.
However there is only a handful of compatible phones and when I signed up they didn't offer any sort of plans. They seem to offer a Nexus 5X for $200 with activation now though. The phone is most likely going to be the biggest problem because of the upfront cost. But if you don't use too much data but want the option to go over without worrying about fees or anything it's by far the best plan out there.