The best way is to see for yourself, try it and cancel if not happy. They all have 30-day rolling plans so make sure to take that (in the shop they’ll try and upsell scam you to the yearly contracts but firmly stand your ground and you’ll get the 30-day one). 
From experience they’re not the best but not the worst either, so worth a shot if the above fiasco didn’t put you off.
I was with O2 until a couple of years ago. For the last six months of my contract with them, I couldn’t use my phone at work as it consistently refused to find a usable signal. That was pretty much the only reason I switched when my contract was up.
I’m now with Vodafone. Their customer service is awful, their website until recently was the most appalling I’ve ever experienced - it would consistently fail at log-in with an Oracle error screen and require long conversations with useless customer service droids to get the account reset - and the only saving grace is that their network covers all the areas I need to use my phone. 
I’ve found Vodafone to have amazing customer service. What route are you using to contact them? I use Twitter and they’ve gone above and beyond in response to even simple requests.
I have no complaints about Vodafone. I just have issues with their latest product, Voxi. For one, I’m not sure age based pricing is really legal (18-25 year olds only), but that’s a minor issue (lots of companies do age based pricing and I sure don’t boycott them all) - the bigger issue is that while appealing on the surface, these plans represent a massive attack on the Internet as we know it. They are a blatant, unapologetic attempt to push the boundaries and see what they can get away with. Net neutrality isn’t something they are even pretending to care about.
I’ve found 02 to be atrocious in high density environments. My office is in the west end, and without the 02 boost box in our office, running over our internet connection (vastly redundant now wifi calling is the norm on phones), there is no physical signal at all. The same can be said for my commute, any high density areas you pass through, even with apparent 4G and full physical signal, the throughput isn’t enough to even send a text.
Unsure what you mean by this?
Ericsson is an MNC that has been manufacturing telecoms infrastructure since practically the dawn of telecoms.
A brief dabble in mobile phones in the 90s/00s made the name more well known before they went into partnership with Sony for the handsets before getting out of mobiles.
Many telecoms companies use Ericsson equipment. AFAIK, Huawei and Alcatel also manufacture telecoms infrastructure.
Ericsson is an MNC that has been manufacturing telecoms infrastructure since practically the dawn of telecoms.
And yet still can’t manage to build decent infrastructure without leaking customer data such as location and completely crashing upon receiving malformed traffic. 
You’ve linked to a 59-page presentation at least partially in French. I don’t know what point you’re making?
My point is that Ericsson (& their friends such as Alcatel) is selling legacy unreliable equipment that is putting at risk millions of mobile subscribers in the world (search for “SS7 attacks” on Google for more info) and you seem to still be defending them for some reason.
And the worst is that mobile operators are buying the crap when they couldn’t be in a better position to build their own and actually make a difference.
You say “magic box”.
I say “telecoms infrastructure”
I’m not defending anything and I have no interest in any debate on this as it seems to have already derailed the original topic.
Indeed let’s keep it at that, but if you are interested I suggest you do more research on the topic and make your own opinion.
[quote=“Merkitten, post:45, topic:29079”]I’ve found Vodafone to have amazing customer service. What route are you using to contact them
[/quote]
Primarily it was though the online web chat. And on one occasion, Twitter, when I vented about their appalling service and they DM’d me.
The central problem, though, was that every time the fix required a hand off to a different team, and I’d have to wait to see if it worked. If it didn’t, or when it failed again, I’d have to go through the same rigmarole all over again, which was deeply frustrating.
t;l;dr, frontline support couldn’t help and had to hand off to a different team, but I had to navigate frontline every single time anyway.