I've used Computer Alliance twice(different machines) on the south side of the OKR just by the flyover & subway exit. I'm not the most technically minded but I've found the two guys in there to be exceedingly reasonable, explaining all the various options in each case including giving the pros/cons of each suggestion. I've not felt pressurised into paying for unnecessary works or even paying for any repair works at all.
the downside is, they can be a bit slow. It's usual to leave the machine with them whilst they diagnose the problems, but they seem to get sidetracked before ringing back to get your permission to go ahead with further work.