My first thoughts on reading this were that it was super-cool. Then I thought about customers who could benefit from it (we provide Linux consultancy to a number of medium-large enterprises), and I pretty much drew a blank.

If you're large enough for the downtime associated with a reboot to be a big issue then you've presumably already got a minimum of two machines performing the same function. That being the case, it's no problem to remove each node from the cluster in turn, upgrade the kernel, reboot, test and add it back in.

That rather makes the service more of a convenience. For example, for active/passive setups where a failover produces an undesirable amount of downtime (say 30 seconds whilst services migrate), or single server setups where uptime isn't critical, but downtime is still a pain.

Cool technology, but perhaps an example of a solution looking for a problem. I think at $4/month the convenience aspect can probably justify the cost in many cases.