there are loads of horror stories regarding kirk frames.
mainly in respect of flex and cracking.
the flexing is an urban myth, but cracking did sometimes happen.
the main fault was failure of the bonnding around the b/b, headset, seat tube etc., where steel shims were inseted into the cast magnesium alloy frame.
i have a couple of mates who still ride their kirks but, to be honest, i would say they would be best suited to a road training bike.
hope this helps.