I'm also experimenting with this. My ADSL link is currently 3Mbps down and < 1Mbps up - its pretty poor.
This week, I got myself a new Three sim on the One Plan, stuck it in a device which doesn't support HSPA+, and get a max download speed of 3Mbps, average of 1Mbps. The upload was terrible at 0.1Mbps.
Today I tried the sim in a Sony Xperia L, which does support HSPA+, I now get a max of 6Mbps, average of 3.5Mbps down and about 2Mbps up.
So its reasonably ok - marginally better than my ADSL. But as others said, there is a fair amount of latency. Speeds are up and down, web pages do take longer to establish its first connection. I guess I need to try it for a few weeks (or months), before I can decide whether or not it can be used it as an ADSL replacement. I'm a light user, I sometimes catch up an episode on iPlayer, so I'm a bit worried about the new TrafficSense throttling my bandwidth.
If it does work, can anyone recommend a decent device to use with this? Three are upgrading to 4G soon, so it might be worth looking for a 4G compatible Android perhaps? Mifi routers are useful, but its also handy to have 2000 mins to use, included in the One Plan.