I'm using Vodafone PAYG with a MiFi. Yes, their website and their customer service is awful and I had a recent experience that was truly horrible. However, once you get it working it works. You can buy your data bundle via the MiFi itself as long as you have credit. For me, buying credit is a bit of a pain. I think you're suppose to be able to do that via the MiFi also, but Vodafone's spectacularly awful website doesn't let me save a credit card for future payments for reasons unknown.
Recently when I tried to buy a data bundle it didn't work, in spite of having plenty of credit -- it turned out if you don't top up at least once per six months you "run out of service days", whatever the hell that means. The result is that you can't log into your account or buy anything, even if you have credit (although you don't lose the credit). Chasing this up was a total nightmare -- I couldn't log in to use any online help, and there was no number to ring customer service, just automated phone menus. Their email support is a joke -- someone replies every two days to ask meaningless questions or offer to ring you on your mobile broadband(!). I eventually resorted to reporting my "phone" stolen, just to get their attention. After I got my top up done, I then tried to buy a data bundle but kept getting timeouts from the MiFi. This resulted me in eventually getting one bundle but being billed for four. After that I had my only positive Vodafone customer experience -- I managed to sign up on their online community forum and the problem got solved very quickly with a prompt refund.
So why do I stick with them? Because the broadband's fantastic. Obviously this is just a fluke of where the house is situated, and I tried all the other providers too but only Vodafone worked. I get an amazing 12 megabits per second from the MiFi and really can't fault it. Prices are: one day/500 MB - €3; one week/2048 MB - €10; 1 month/5120 MB - €20. PAYG is great for me -- I often spend only a day or a week there, and was previously spending €60/month on satellite broadband. What a pity Vodafone are such an unbelievably awful company to deal with.