I've bought a ton of stuff from Mikes, mostly little bits and pieces. had problems with only a few:
* clutch lever - didn't fit with 650central cable, does not have enough pull/adjustment to properly disengage clutch. stock lever worked fine.
* stainless brake lines - they claim some degree of swivel on the end for fitment, but they do not swivel at all, which means you have some funky angles and twist involved and they are hard to put on. this was before I knew aluminum fittings were bad to use, so I'll be ditching them all together now.
* brake master cylinder/lever - this one has a standard thread hole for mirror, not LH thread like most yamaha right side levers. if you have yamaha mirrors already, the site recommends a thread adapter set, which I ordered. however, the adapter set is for putting a standard mirror on a LH thread lever, not a LH thread mirror on a standard lever, so it doesn't work. he took the return back after I explained, but the site still has the wrong recommendation last I checked. the brake lever itself works fine as far as I can tell.
i've mostly had problems with ordering parts that won't work together, like the levers/mirrors or spark plugs with the wrong plug caps and such. my fault for not knowing ultimately, but probably easy for someone who sells this stuff to spot when they fill the orders.