The NVMe driver will only work for a fairly narrow set of uses in which the whole NVMe device(s) can be dedicated to a single application (this is because the user-space application takes control of the NVMe device directly, so the kernel driver can't simultaneously use it).
Some of the straightforward use cases would be inside network-attached storage appliances (ideally in conjunction with a user-mode network stack) or in a database (database systems already typically want to avoid any OS interference with storage access). In general, the NVMe driver can be dropped in fairly easily when existing code is using something like Linux AIO with O_DIRECT on a raw block device; the AIO programming model maps quite directly to the NVMe driver programming model (create a queue, submit I/Os, and poll for completions).
Some of the straightforward use cases would be inside network-attached storage appliances (ideally in conjunction with a user-mode network stack) or in a database (database systems already typically want to avoid any OS interference with storage access). In general, the NVMe driver can be dropped in fairly easily when existing code is using something like Linux AIO with O_DIRECT on a raw block device; the AIO programming model maps quite directly to the NVMe driver programming model (create a queue, submit I/Os, and poll for completions).