What's your opinion (based on experience)?
There's no harm in taking it (as long as you can be bothered with it) - language schools do tend to build up networks of trusted partners, and I managed to get quite a lot of work last year on the basis of simply being known to one school in Warsaw - whenever they needed a native, they always checked with me first. They had absolutely no presence in Poznan, but they were getting companies signing up for classes in Warsaw - and they'd often want classes in Poznan, too.
It can't hurt you really - many big companies will only deal with big schools. I know one example where a company was forced (by their foreign bosses) to have classes with Profi-Lingua, not because they were good, but because they were nationally known. Utter idiocy to pay well over 100zl an hour (the factory was nowhere near the school) for a teacher, but so be it.
And while you might not steal the client, there's no harm in using it to promote yourself if they want extra classes. I always lived by the philosophy that the more contacts you make, the more work you can find.
I think I'll take it on the condition their methodology director doesn't say or do anything to p*ss me off.
Seems to be an almost universal trait - never have I met more self-important people than methodology directors. I think I've only worked with one who actually understood her job - to be available if needs be, to run training sessions and to keep up to date with things. She also understood the meaning of "light touch".
I really should get round to posting a thread about schools (and people) to avoid in Poland. One amusing story involves a certain school director paying his teachers in deserted car parks late at night in cash!
The only thing I'd be wary of is the contract - don't be afraid to tell them to remove chunks of it.