I've signed up, can only give it a go sounds OK to me, I have fast internet here in the UK but sounds like a lot of people are going to struggle to reach 35mbps.
With Stadia, you can stream up to 4K, 60fps, with 5.1 surround sound, at a minimum internet connection of 35Mbps. On Google's end, it's using the following hardware specifications:
Custom x86 processor clocked at 2.7GHz w/ AVX2 SIMD and 9.5MB of L2+L3 cache
Custom AMD GPU w/ HBM2 memory, 56 compute units, and 10.7TFLOPs
16GB of RAM (shared between CPU and GPU), up to 484GB/s of bandwidth
SSD cloud storage