In my recent travels I have noticed that some wireless network operators have greatly improved their network for Internet access while others have consistently declined. To examples from both ends of the scale:
I haven't used the network of Wind in Italy for notebook access to the Internet a lot in the past anymore since they had no HSPA in their network. Even in the standard 3G mode, connections where slow due high packet loss. Recently, however, they have upgraded their network to HSPA, at least in Rome, and I have since gone back using their prepaid SIM for notebook Internet access. I consistently get around 1 MBit/s in downlink direction (most likely traffic shapped) and 384 kbit/s in uplink direction when sending eMails with file attachments and pictures. Well done Wind!
On the other end of the scale is 3UK. I've bought one of their SIM cards a couple of months ago and during that time, their performance in the UK was ok. These days however, both in the UK and abroad, I consistently get bad throughput and sometimes the connection doesn't work at all. During a week in London last week I got so frustrated with their service that I stopped using it at some point and replaced it with another prepaid SIM. Randomly blocked TCP and UDP ports to keep me from getting eMail and setting up my VPN connection in addition to slow throughput is not acceptable. This week in Rome it's also been pretty much unusable, data rates are just too slow. Well, I guess I will try again in half a year. Not earlier probably, because I don't see a reason to waste 10 pounds to activate the data option just to find out their service is anything but a service…
For alternatives and other countries take a look at the prepaid wireless Internet access Wiki.