3 are great - until there is a problem
I've been on 3 for a couple of years. Originally did it because they didn't charge roaming charges on other 3 networks. (They soon got rid of that! ).
I'm a big user of data. It is 3's data allowance (2GB) that keep me using 3. My only beef with them is their support staff and support policies.
On 3 different handsets I've suddenly lost 3G signal and I need to flip to flight mode a few times to get it back. The support line are abysmal. They wont help me and they are unbelievably patronising at time. Why won't they help me? .... Well my biggest beef is that 3 DO NOT SUPPORT YOU IF THE HANDSET IS NOT '3' BRANDED. I have been buying sim-free handsets from Amazon and Handtec for a number of years for both my partner and family. It's the only way to get decent handsets from HTC rather than waiting AGES for 3 to provide the same handset.
'Frank' from 3 (obviously not his real name) told me that "3 install special software on their handsets to make them compatible' and as a result , unbranded handsets (e.g. those without the 3 logo) can not be supported. What absolute tosh. No amount of reasoning with the clowns on support helps you in this situation.
So, yes 3's network might be OK, but you're on your own if your handset was not purchased through them. You get diddly-squat support. HTC ought to have a word....