I can so far only comment on the two I have used between Dundalk and Dublin. I have used a vpn client to connect to my work with the three modem and with a good signal it was quite good. The main dropout areas I have found are between skerries and Balbriggan, sometimes between howth Junction and malahide - especially coming into malahide, and two other spots between Drogheda and Dundalk - one between Drogheda and dunleer and then about 5 minutes from Dundalk, this one seems to affect quite a few carriers though, inc O2.
On the roaming front.. 3 is actually very good!! I've used the 3 modem in NI, Scotland, Wales and london and had excellent coverage and speeds compared to Ireland and even on lake Garda in Italy
and their 3 like home policy applies for broadband usage in countried that they have a presence which meant it came out of my monthy allowance/costs!