Networks such as giffgaff and iD Mobile might not be as well known as Three, Vodafone or O2. But that's not to say that the service will be worse.
That's because these so-called challenger networks' service is in fact based on infrastructure that's owned and operated by the larger, household-name networks.
That means that giffaff, iD Mobile et al are able to offer the same sort of speeds and coverage as their competitors.
What you may miss out on with less well known networks are freebies and customer incentives that O2, EE and more are able to offer. But given that lesser known networks tend to be cheaper, you may decide that you won't miss them.