There was an old company called SIM4Travel, the only Global SIM that actually worked well, it was fantastic, it was cheap, then one day I got a very sad email saying they're shutting down and they got bought by TruPhone or something like that and I couldn't keep the UK number they had assigned to me and had to get a US one instead. So I cancelled my subscription and went on looking for a new one.

Came across WorldSIM since I needed something that provided UK numbers, ordered a SIM card, absolute and utter crap. It works somewhat well in England, where it latches on to O2-UK instantly, but outside UK (which was the main purpose of me buying one), it barely ever works at all.
My dad went to Italy and was using it there, I would call and it would ring for me, however nothing on his phone. Same for Germany, only texts would go through, albeit an hour or so late, but calls were few and far between.
Then, in UAE where I travel to a lot, it doesn't work at all, despite having both carriers listed as roaming partners, it refused to latch on to either network.
I tried different phones, still nothing. Just stuck on 'Emergency Calls Only', I took it up with their customer support team who replied a week later saying you're only allowed to use it on roaming in UAE while on 2G (GSM) Only mode. WCDMA (3G) will not work, however neither worked. I tried WCDMA/GSM Auto, WCDMA only and GSM only. It would show both networks in the search menu every time, but would never actually connect to one. I went back and forth and they kept insisting my phone was faulty and everything checks out at their end. So, I just left it, if it doesn't work in Europe and it doesn't work in the UAE. Where exactly does it work, other than the UK?

Before all this bad experience however, when I received the SIM card, but hadn't used it yet, I recommended it to a relative. Damn, do I regret that. They kept asking me for tech support help then. My aunt ordered a SIM from them with a US based number to use in the US and Canada on her trip. Texts would take forever to get sent and they would be delivered to the recipient hours later. Calls, although a bit choppy, would go through fine, both in US and Canada.

In short, WorldSIM sucks, don't get it. Wasted about 40 pounds of credit.

As other members have said, it's much easier to just buy a local prepaid SIM Card and use that instead.