A while ago I bought a Vodafone USB stick/dongle/modem while in England (a Huawei K3565). Has served me perfectly over there. I chose the Vodafone modem, because it came with drivers that work for Linux, which I happen to use.
Vodafone had a perfect plan as well: top up only, so no huge bills. Roaming charges are horrendous though, really making it unusable. So while in Israel I bought a Cellcom sim. Unfortunately it didn't appear to work. The salesperson suggested a SIM lock, and this is indeed the case. I requested a SIM unlock code, but after three days hadn't received anything. Called Vodafone again, and they said it they handn't even processed the request, and it would take 10 working days!!!
After some research I found a reputable company that produced an unlock program that worked! The only problem with this program is that it is Windows only. I own a copy of VMware workstation, so I used that to unlock the stick.
PS: when connecting to Cellcom using the NetworkManager, don't use any username/password, and use "internetg" for the APN.