Strange timing, as I've just made the very same switch. Instantly I'm getting 3/4 dots where I live, whereas Vodafone struggled for one - despite living on top of a hill. I've Not taken my phone all the way up to Waterloo yet but it seems to do rather well up to Wimbledon on the occasions I've made that journey - Only once fading a little in the Brookwood area, and going the opposite way on the SWML to Southampton it has been fine - including 3 dots of signal in the previous blackspot of Micheldever. My Three IPhone also worked rather well over on the Island of Wight recently too, all in all a move I'm rather pleased to have made. Coupled with SWT's rather good free Wifi, it's all a thumbs up from me so far!
Haha. For me it was the 2GB data for £10/month (with 200 minutes and unlimited texts) that made me switch. The speed of the data seems faster, however it only works on H+ and not on H or 3G, which is the issue I have at Waterloo :? (I have the S3 so not a 4G phone). Ah yes Micheldever was also problematic, did have Winchester in my mind. I've not had issues with the wifi the few times I've used it, even when one of those was a single 444 to Portsmouth that had all seats taken.