It's not rubbish as a couple of people have said though I suppose it depends on your own experiences. You can get quite reasonable one-dish meals for under a tenner that are tasty and filling. If you like rice/noodle dishes, it's worth trying, but be warned - as has been said - they don't have conventional tables only long bench-style tables so if there's just one or two of you, you get put on the end of one of these. At less busy times you shouldn't necessarily end up sharing with other parties.
I've enjoyed it in the past but feel like I've "done" Wagamama now. This was underlined by visiting the London South Bank branch and being served warm lager on a day during the hot spell. They seemed perturbed by my mentioning it, and were unable to find a cold one. And whether the meal was coloured by that I don't know, but the noodles I usually enjoy seemed tasteless, production-line-like and tepid. But bear in mind each branch has different management so some will be better than others.
I must admit I get a bit wary of chains because they're uder orders to reproduce everything to match every other chain as far as possible and when food gets forced into those sorts of strictures I think it loses something.