As a breakdown engineer, they're all rubbish. So doesn't really matter too much what you fit. It'll break down. But from what I have seen, the Baxi products from their HE range, not the GA range, are the better ones to go for. They break down yes, but the fault is usually something very simple, not like WB needing fans & gas valves. Haven't worked on any Logics, but I think everyone has been put off Ideal after the disaster that was the Isar.
Someone also mentioned the GW Ultracom 2 on here being good. They've got more plastic than the Worcester & IMO aren't much good. They don't burn as clean as the Baxi products so either.