Not with me. I get better speeds up and down on Three than i did with giffgaff (run off o2's network) and a better signal.
I do as well, O2/giffgaff weren't too hot for me either. Three were IMO a million times better than all others.
I just find them to be getting worse. It's is not guessing etc...it's what is happening to me. Noticing this down in London and up in the north east. As are others who moved to three around the same time.
If it works well where you are though that's great and I hope it stays that way.