National XT4000 tires, can I live with them?

So, I bought my well-used Prius last Thursday, and so far I love it except for the rear tires. It has Hankook H724's on the front, which I have no problem with, but the rears are National XT4000, which apparently is so cheap and generic a tire nobody has even bothered to review it, as far as I can find. So far I haven't had any traction problems with them in dry or rain, and I seem to be getting ~50mpg despite the way I drive (pending proper calculation next time I fill up) but they're noisy to the point that I don't even know how tires can be that noisy. Also I'm not sure they're quite as round as tires really should be.

I was looking at some Michelin Energy Saver A/S to replace them, but dropping ~$340 (after installation, etc.) on two tires is really not in my budget for at least a few more paychecks, unless there's a safety concern.

So, has anyone had experience with these? Are they probably going to explode on the highway at some point?