I work on yachts and we have a company called
www.gymsim.com that supplies us with a 423 SIM and also with a 44 sim.
For yacht crew the 44 seems to be a good product because of the free roaming in both Europe and the Caribbean, but the system has been pretty spotty - though we have been promised many upgrades in the next few weeks, (I have to give credit to gymsim support, there is a lady there called Sonia who lives to talk to the crew and find out how we are doing and tell us whats happening!!!) they just had a switch upgrade last week. Aparently signalling is one of the big problems, their current signalling/roaming provider is BPL out of india (Is this who Xius use?), and they arent so good.
The new provider is supposed to be TIM out of Italy which they say will make a big improvement...will see how that works out next month.
Texting is also not the best, not sure what is happening with that.
I am still using my 423 number, but thinking to use it less when the 44 number becomes more reliable.
I used my 44 SIM in the following countries, and was usually able to get network connection most of the time, but you have to fight it out, manual network selection, changing networks turning phone off and on - its a royal pain in the ass, but you can do it.
St Martin
Antigua
USA - OK in Florida, had problems in NY
UK - strangley I found it worked on auto network selection better then manual.
France - OK (but can only connect to Orange)
Italy - OK (but doesnt like Vodafone - drops out sometimes)
Croatia
Spain
Greece
Turkey - had lots of probs with turkcell, but was able to connect with cosmote when close to greece.
But yeh, the system needs improvement, may calls sometimes just drop out too - in the meantime im loving my 423, tho you find some people just cant call to the 423 number.