What makes it "great" and a better choice than an AC88U?
Hardware accelerated AES-NI has a big impact on VPN performance. USB disk sharing also has much better performance for anyone using it. Based on Broadcom's newer platform, which will most likely get more active long-term support from Broadcom than the old SDK7 used by the RT-AC88U. The switch to glibc instead of a prehistoric version of uclibc also resolves a number of esoteric quirks that can affect general stability. Newer Linux kernel (4.1) which has received a lot of IPv6 fixes over the old 2.6.36 version used by the RT-AC88U.
The overall software stack is much more modern, and provides a lot of improvements over the old one used by the RT-AC88U.
The RT-AC88U is a tough sell, because in itself it barely brings any improvements over the RT-AC68U, while being significantly more expensive. I expect the RT-AC88U to gradually phase out, its mid-range spot being taken by the RT-AC86U.