I live in Canada but, whilst in the U.K. picked up a cheap GSM SIM card for my nice little Treo 600. The intent is to avoid double-transatlantic billings; if I'm in the UK and somebody calls my Canadian (Rogers) number, the caller gets nicked for long distance TO Canada, and I get billed for long distance from Canada back to the UK. Hence the SIM, which seemed to work fine in my Treo.
There was some problem with registering it on their web site but, to my surprise based on what I'd since read about their customer service, they actually phoned me from the UK to straighten it out. I see elsewhere on the web people having complaints about VM, but most are hardware issues with particular phones. From what I've seen, VM are doing a good job of what they're at, which is low-frills, youth-oriented (didn't know that when I bought it) mobile phone service. At least in the U.K. In Canada, they're apparently planning to partner with Bell, which uses a different technology (CDMA). This may be good for sales but won't help their UK customers roaming here...
Now their web site itself is nice, but the performance isn't there. So what sucks is probably strategixsoftware.com, the whiz-kid enterprise software providers, or perhaps the people at VM who did (or didn't do) the web site capacity planning.