I think Five Guys is really excellent. The fries aren't soggy because they are badly done - that's just the way they do them. If it's not for you- it's not for you.
The prices are on the high side, but it's comparable to other things in the market like Nando's. £1-2 either way isn't really a big deal when you're in town and they are offering free refills.
If you like crispy fries, the one to try is Dirty Burger in Kentish Town. Be warned - the physical environment is really basic.