I've been with VM and Telewest for 25 years. When it works it is excellent. When thing go wrong, or you need to contact them, even for something that should be simple, it can be simple.....but too often it can be a nightmare. VM customer service moves in long cycles, and we're currently at a very low point. Behind the scenes, VM's installations, technical support, customer service and complaints processes are all deeply flawed, staff operate in silos that don't communicate, and there's no proper online capability, with the company wanting customers to use some of the worst offshore call centres I've ever come across. They've recently introduced "omni-channel" customer service that supposedly works across voice calls, chat, and Whatsapp. In reality it's just made things worse, with more opportunities for agents to redirect or lose calls. VM never see the world through their customer's eyes, and that how VM earns their well deserved reputation. Wifi (wireless) performance of VM hubs is mediocre - in an easy wifi environment they're OK, anything remotely challenging and you may find you need to consider investing money to resolve that.
If your current problem is a connection that's slow, but has good latency (ping), choose carefully, as the technology VM use is measurably worse than Openreach for latency, and prone to spiking for some customers. Downloading a game in no time is great, but if you find that playing it is worse you might regret that.
If you need the speeds, then give it a go - an eighteen month contract could well tide you over until there's a modern FTTP option available. And you may be connected quickly, have a good quality connection, and think "what was all the moaning about?" But remember that you have a cooling off period of 14 days starting from when your connection goes live - after that 14 days you're locked in for 18 months with draconian early termination fees unless you join on the pricey 30 day rolling contracts. I think on balance I'd recommend the 30 day rolling if you can afford that. If it's all good and you think you want a better value deal, you could phone to cancel the rolling contract, and the retentions agents would find you a discount in return for an 18 month fixed term contract.