I have the normal iPad and iPhone. It works really well but there is one other thing to be careful about.
On the cellular you can select which apps are allowed to use data on 3G/LTE and which can only use data on wifi.
If you use the iPhone as a hotspot it looks like wifi to the iPad and it will download app updates, newstand subscriptions and other such things. Could be a problem with a limited data plan.
I usually turn off the hotspot on my iPhone as soon as I am done to prevent this. Also I don't have any subscriptions that auto-download. App updates are still a problem and I like keeping those to auto download from wifi.