Having owned around 10 aeroplanes thus far, half PFA, half CAA, the majority of horrors have come from M3 establishments.
That said, I know of many establishments who are thoroughly diligent and offer superb customer service so this is not a knocking post.
Worst offenders - maintenance organisations attached to flying clubs - keep them flying, spend nowt !
I've had machine screws replaced with self tappers, a fabric repair signed into the logbook as satis - it was silver gaffer tape, sewn on. An inverted valve seized solid - the aeroplane had just been given a new C of A and flight tested !
On the PFA side, I had one aeroplane with a fresh permit, after the delivery flight I noticed a bit of oil and a nasty exhaust smell - the manifold on No3 was cracked 40% round and there were 4 serious oil leaks.
I have been fortunate to work with a very good engineer for the last few years, we have a good relationship, I work, he checks and signs and I pay for his time. Not everyone is able or willing to do this but it certainly works well for some.
Get a recommendation, visit the premises, talk to the mechanics and the engineer - don't forget, there are probably only 1 or 2 guys who sign the stuff off, the rest are paid mechanics with varying amounts of experience.
If you are not happy, write and document everything and don't be afraid to consult the CAA if you don't get a reasonable result.