I just received a new Probook 4545S today, and I decided to try the Windows 8.1 upgrade on it.
Unimpressed with 8.1, I opted to recover the computer to its factory state using F11.
Well, I can see the recovery partition, but apparently 8.1 made some changes that affect the ability to recover the Windows 8 original configuration.
It shows 8.1 as a selectable OS, but even that doesn't work when its chosen.
Not wanting a system without a working recovery partition, I chatted with HP support, and they are sending me a recovery DVD set for Windows 8.
This may or may not be a big deal to you, but it's information worth having.
If 8.1 renders my factory recovery unusable, it may well do the same to yours, so at the very least make a recovery set before upgrading.
Besides all of that, the Start button that 8.1 adds only launches the tiled desktop. Only Classic Shell gives you a true Start menu.