Never heard anything bad about JKM, which to be honest is a rare thing (I get to hear most of the horror stories about VAG tuning companies). As Robin said though they aren't the cheapest so when it comes to having expensive parts fitted you may do well to try to source certain parts yourself and just pay them the labour charge.
....Just to clarify, I wasn't implying that JKM are more expensive than most but only that they aren't always the lowest price you can get by shopping around.
To illustrate this, JKM were only about £20 (on £1,500) more expensive than APS and about £30 cheaper than VWR for supplying and fitting a Quaife. I only went for VWR because of their experience and JKM were totally honest in telling me that although they had fitted many many diffs, not yet in a DSG. When sourcing some Ferodo DS2500 pads, JKM were cheaper and faster in supplying than AP Racing's recommended cheapest supplier.
Depending on which parts I would strongly advise
against sourcing your own parts and supplying them to
any workshop without first discussing it with them. You can't expect any workshop to take responsibility or chase up any product warranty issues unless they have supplied them.
It's never worth being a cheapskate IMO.
