Those that have installed them on here seem to think that the Logics are better than previous efforts from Ideal, which have at times been abysmal heaps of junk. There are better boilers out there but it of course depends on your budget.
As for whether 18kw is enough, this rather depends on your individual property, there is no such thing as a 'standard' house/bungalow/mansion etc. Among other things the level of insulation and the total floor area play a part in determining what size boiler you need. Your installer should have taken measurements and notes and calculated the boiler size required.