802.11ax, otherwise known as Wi-Fi 6, is due to be formally ratified sometime in 2019. In the meantime, many vendors are shipping "conformant" products branded as 802.11ax. They can do this because these companies have representatives on the IEEE committees and know that the majority of the specification is frozen, and it is only small tweaks remaining before the standard is ratified. These companies also know that any changes made between now and when it is ratified will be small, and will not make any of the existing silicon in their equipment obsolete - changes from now on can be captured in code/firmware changes.
So what does Wi-Fi 6 give us? Everyone talks about speed, with the standard supporting up to 1.1Gb/s on the 2.4GHz band (with all four spacial streams up and running) and 4.8Gb/s on the 5GHz band (with eight streams). However, to achieve these speeds (which, remember, are RF speeds, not data transfer speeds) you are going to need brand new Wi-Fi 6 Access Points, and brand new Wi-Fi 6 client devices, both with the full complement of chipset, firmware and aerials, and a good, clean RF environment. Also, you need a way to connect the Access Points to your distribution network to support the burst data rates, a good connection from your distribution network to your core, and applications that can deliver the data to the end users at that rate (how many web connections do you know that need 4.8Gb/s of bandwidth to fill the screen.
The above paragraph is very simplistic, because other benefits of Wi-Fi 6 come to the rescue, with enhancements like the ability to support concurrent wireless sessions to multiple devices in the same time slot (real MU-MIMO) and Orthoganal Frequency Division Multiple Access (OFDMA) to connect to multiple devices at the same time, as well as smarter beamforming. All of these extra enhancements mean that more end user devices can connect at the same time, and share the RF (which IS big news, given RF is usually a single user, half duplex environment), delivering a better service to the business (or home).
Will you notice the difference? Probably not for the next 18 months, until a critical mass of both Access Points and client devices get out there that support the Wi-Fi 6 standard. Also, the performance figures given above are best case, fully loaded, 8x8 device numbers, and there won't be many of those around initially - and they won't be cheap. They are not the sort of equipment that will turn up in home/soho routers quickly.
Caveats? Remember one of the basic rules of wireless network design - the wi-fi is just one hop in the network chain. It's not going to help installing the latest and greatest Access Points in a branch office if the access switches can't provide them with the multi-gigabit connections, or especially if the backhaul to the datacentre/cloud is limited to 100Mb/s, 50Mb/s or 10Mb/s - 2Gb/s at the AP down to 50Mb/s to the cloud doesn't really work. Don't blame the wireless network if the backhaul can't cut it - be careful in site upgrade scenarios where the customer will expect huge improvements in end user performance with the new gee-whiz wireless, when the backhaul is the issue. Manage the expectations.
When should you start installing 802.11x APs? Now! The installed life for commercial wireless gear is about five years, and within that five year period there WILL be lots of client devices that support Wi-Fi 6. Some of the larger smartphone manufacturers are saying this year's crop (2019) will contain Wi-Fi 6 chipsets. APs installed now will need to support Wi-Fi 6 client devices in their lifetime, and you don't want your customers to accuse you of selling them obsolete equipment. However, don't forget to understand the business requirement before you just install Wi-Fi 6 equipment. In a normal office environment, with smart devices and laptops everyone, Wi-Fi 6 is appropriate. In a factory/distribution environment, where smart devices are likely to be rare, and handheld scanners and IOT devices the norm, Wi-Fi 5 is likely to be overkill, let alone Wi-Fi 6.
Bottom line - design to the requirements, but consider the future.