On the negative side I thought my exhaust had sprung a leak after taking it to KwikFit for MOT and service until I took it to a local garage (should've done that to begin with) and they found that they hadn't screwed the air filter cover back on, it was just sitting there rumbling away.
What was even better was that there were two screws nestling down an inaccessible nook by the side of the engine. Maybe they tried to re-fix it, failed and thought "sod it"?
Have had positive experiences of them before though, think it's heavily branch-dependent…