USI-enabled styli have more benefits than cross-compatibility. User preferences are saved in the styli themselves, so color choices and ink styles can be automatically carried over from one device to another. Multiple users can write or draw on a device simultaneously, making USI-compatible tablets and computers ideal for collaboration (or maybe even multiplayer games). Plus, USI says that partners can build out additional features.
As for the what HP is announcing, CNET reports that the new x360 Chromebooks will be available this week in 12-inch and 14-inch models, priced at $359 and $379 respectively. While the price difference is negligible, there are differences that may steer you in one direction or the other. The smaller model has an Intel UHD Graphics 620 chip and 12 hours of battery life while the larger one has an Intel UHD Graphics 600 chip and 10 hours of battery life. Both feature the same Intel Celeron dual-core N4000 CPU, USB A and C ports and a magnetic stylus dock. The $70 USI Pen stylus won't be launching alongside the Chromebooks, though -- it won't be out until November.
We'll likely see more USI-enabled devices shipping soon. Major partners like Google and Intel are currently working on USI products, but HP's USI Pen is the only significant one slated to hit shelves. A single-stylus solution certainly sounds a lot more appealing than carrying around one per device, so hopefully more products will be released soon.